
Class 
Book 



r 











Copyright N°. 



COPYRIGHT DEPOSIT. 



HEAT-TREATMENT 
OF STEEL 



HEAT-TREATMENT 
OF STEEL 



A COMPREHENSIVE TREATISE ON THE 
HARDENING, TEMPERING, ANNEALING 
AND CASEHARDENING OF VARIOUS KINDS 
OF STEEL, INCLUDING HIGH-SPEED, HIGH- 
CARBON, ALLOY AND LOW-CARBON 
STEELS, TOGETHER WITH CHAPTERS 
ON HEAT-TREATING FURNACES AND ON 
HARDNESS TESTING 



FIRST EDITION 



NEW YORK 

THE INDUSTRIAL PRESS 

London: THE MACHINERY PUBLISHING CO., Ltd. 
1914 



TS3?0 
.H38 



Copyright, 1914 

BY 

THE INDUSTRIAL PRESS 
NEW YORK 



OCT 3 1914 



Stanbopc ]press 

H.GILSON COMPANY 
BOSTON, U.S.A. 



CI.A379842 



iy-i6tf} 



^ 



PREFACE 



In the development that has taken place in the methods and 
processes pertaining to the machine building trades during the 
past fifteen or twenty years, most remarkable changes have been 
wrought in the heat-treatment of steel, including the harden- 
ing, tempering, annealing and casehardening of the various 
kinds of steels. The introduction of high-speed steel and of the 
various alloy steels has especially demanded great modifications 
of past practice. The present book places on record the modern 
methods now employed in the heat-treatment of steel, and in- 
cludes also a treatise on the methods used for measuring the 
hardness of metals by the various hardness testing apparatus 
that have been developed in this country and abroad. 

Special attention has been given to a number of methods very 
recently developed, making this book the most modern and 
complete on the subject; thus, for example, a very comprehen- 
sive treatment is given of electric hardening furnaces, a develop- 
ment unknown only a few years ago. Another of the more 
recent developments to which attention has been given is the 
method of casehardening by carbonaceous gas which has been 
developed very recently. 

The well-known twenty-five cent Reference Books which 
Machinery has published since 1908 and of which one hundred 
and twenty-five different titles have been published during the 
past six years, include the best of the material that has appeared 
in Machinery in past years, adequately revised, amplified and 
brought up-to-date. Many subjects, however, cannot be cov- 
ered to an adequate extent in all their phases in books of this 
size, and in answer to a demand for more comprehensive and 
detailed treatments on the more important mechanical subjects, 



VI PREFACE 

it has been deemed advisable to bring out a number of larger 
volumes, each covering one subject completely. This book is 
one of these volumes. 

The information contained in this book is mainly compiled 
from articles published in Machinery and the best on the sub- 
ject that has appeared in the Reference Books is also included. 
Amplifications and additions have been made wherever neces- 
sary. For the material contained, Machinery is indebted to 
a large number of men who have furnished information to its 
columns. In many cases it has not been possible to give credit 
to each individual contributor, but it should be mentioned that 
some of the most important chapters have been mainly compiled 
from articles by Ralph Badger and E. F. Lake. To all other 
writers whose material has appeared in Machinery and is now 
used in this book, the publishers hereby express their appreci- 
ation. 

Machinery 

New York, June, 1914 



CONTENTS 



Chapter I 
HARDENING CARBON STEELS 



Pages 



Effect of Difference in Composition of Steels — Effect of 
Heat-treatment — The Hardening Process — Critical Tem- 
peratures — Decalescence and Its Relation to Hardening — 
Quenching — Theory of Critical Points — Apparatus for De- 
termining Critical Points — Tempering — Use of Pyrometers 
— Brayshaw's Experiments on the Heat- treatment of Steel — 
Length of Time of Heating — Effects of Previous Annealing — 
Effect of Heating in Two Furnaces — Effect of Tempering. . . . 1-25 

Chapter II 

HEATING THE STEEL FOR HARDENING 

Simple Type of Gas Furnace — Oil-burning Furnaces — 
Characteristics of Fuel Oils — Kerosene for Steel Heating Fur- 
naces — Modern Types of Gas- and Oil-fired Furnaces — Ad- 
vantages of Oil and Gas Furnaces — Electrically-heated Fur- 
naces — Solid Fuels for Steel Heating Furnaces — Heating 
Steel in Liquid Baths — The Lead Bath — Cyanide of Potas- 
sium Bath — Barium-chloride Bath — Pyrometers for Gaging 
the Heat — Calibration of Pyrometers 26-60 

Chapter III 

QUENCHING AND TEMPERING 

Kinds of Quenching Baths — Receptacles and Tanks Used 
for Quenching — Tempering by the Color Method — Baths 
Used for Tempering — Tempers for Carbon Steel Tools — 
Tempering Furnaces — Defects in Hardening — Overheated 
Steel — Scale on Hardened Steel — Annealing Steel 61-80 



V1U CONTENTS 

Chapter IV 
HEAT-TREATMENT OF HIGH-SPEED STEEL 

Pages 

Hardening High-speed Steel — The Taylor- White Process — 
Tempering High-speed Steel — Annealing High-speed Steel — 
Heat-treatment of Different Kinds of High-speed Steel 81-92 

Chapter V 

HEAT-TREATMENT OF ALLOY STEELS 

Nickel Steels — Nickel-chromium Steels — Chrome-vana- 
dium Steels — Casehardened vs. Oil-hardened Gears — Neces- 
sity of Heat-treatment of Alloy Steel — Composition, Heat- 
treatment and Properties of Carbon and Alloy Steel 93~in 

Chapter VI 

HEAT-TREATMENT OF STEEL BY THE ELECTRIC 
FURNACE 

General Remarks on Heat-treatment — Electric Heat — 
Description of Electric Furnace — Advantages of Electric 
Heating — Practical Application of Electric Furnaces — Heat- 
ing Gears in the Electric Furnace — Action of Carbon in 
Heated Steel — Effect of Oxygen on Iron and Steel — Effect 
of Nitrogen on Steel — Current Consumption and Operating 
Cost of Electric Furnace 112-127 

Chapter VII 

METALLIC-SALT BATH ELECTRIC FURNACE 

Requirements of Hardening Furnaces — Description of 
Hardening Furnace — Current Consumption in Salt-bath 
Electric Furnace — Temperature and Composition of Hard- 
dening Bath — The Hardening Process — Advantages of 
Salt-bath Electric Furnaces — Results Obtained in Electric 
Furnaces — The Use of Barium-chloride for Heating Steel 
for Hardening — Barium-chloride in the Electric Hardening 
Furnace 128-145 



CONTENTS ix 

Chapter VIII 
MISCELLANEOUS TYPES OF ELECTRIC FURNACES 

Pages 

Furnace Used for Heat-treating Leaf Springs — The Salt- 
bath Furnace — Melting Points of Different Salts Used for 
Heat-treating Steel — Electric Arc Heating — Local Harden- 
ing by Use of Electric Heat — Rapid Method for Hardening 
Steel by Means of Electric Heat 146-156 

Chapter IX 

MISCELLANEOUS HARDENING METHODS 

Pack-hardening — Packing Materials — Methods of Pack- 
ing — Heating the Steel — Pack-hardening Gages — Pack- 
hardening Hammer Dies — Furnaces for Heating Hammer 
Dies — Methods of Cooling — Cooling Baths — Tempering 
in Oil — To Prevent Scale on Dies when Hardening — Heat- 
treatment of Dies and Tools Used in Forging Machines — 
Heat-treatment of Vanadium Tool Steel — Hardening the 
Heads of Forged Tools — Heat-treatment of Spring Steel — 
Heat-treatment of Screw Stock — Heat-treatment of Steel 
Castings — Hardening Cast Iron — Local Hardening 157-179 

Chapter X 

CASEHARDENING 

General Principles of Casehardening — Steel to be Used for 
Casehardened Parts — Hardening-room Equipment — Case- 
hardening Furnaces — Boxes for Casehardening — Packing 
Materials — Packing the Boxes — Carbonizing — Methods for 
Ascertaining Proper Carbonizing Temperatures — Reheating 
and Hardening — Reheating Furnace — Quenching Baths — 
American Society for Testing Materials Methods of Case- 
hardening — Results of Hardening without Reheating — Case- 
hardening in Cyanide — Local Hardening — Casehardening 
Alloy Steels — Degree and Depth of Hardened Surfaces — 
Cleaning Work after Casehardening — Straightening Work 
after Hardening — Casehardening for Colors 180-210 



X CONTENTS 

Chapter XI 
NEW CASEHARDENING METHODS 

Pages 

Development of Casehardening by Carbonaceous Gas — 
Comparison between Old and New Methods — Kind of Car- 
bonaceous Gas to Use — The Giolitti Process — Operation of 
the Giolitti Furnace — Effect of Compressing the Gas — Time 
Required for Operation — American Gas Furnace Co.'s Appa- 
ratus for Casehardening by Gas — Steel to be Used for Gas 
Casehardening 211-225 

Chapter XII 

HEAT-TREATMENT OF GEARS FOR MACHINE TOOLS 

Casehardened Gears — Hardened High-carbon Gears — 
Comparison of Results by the Two Processes — Furnace for 
Heat-treating Gears — Quenching Baths — Drawing or Tem- 
pering Bath — Application and Calibration of Pyrometers — 
Cost of Heat-treatment Equipment 226-235 

Chapter XIII 

TESTING THE HARDNESS OF METALS 

Importance of Hardness Tests — Definition of Hardness — 
Simple Methods of Hardness Testing — Modern Methods of 
Hardness Testing — Turner's Sclerometer — Shore's Sclero- 
scope — Brinell's Test — Keep's Test — Comparison between 
Testing Methods — Hardness Test on Worked or Heat-treated 
Metals — Hardness Scales Compared — Hardness of Steel in 
Hardened, Tempered or Annealed Condition — Relation be- 
tween Hardness and Wear of Steel — Details of the Brinell 
Method for Hardness Testing — Relation between Hardness 
of Materials and Ultimate Strength — Machines for Testing 
the Hardness of Metals by the Brinell Method — Derihon 
Portable Form of Hardness Testing Machine — The Ballentine 
Hardness Testing Device — The Shore Scleroscope — The 
Shore Hardness Scale — Uses for the Scleroscope — The 
Scleroscope Applied to Shop Work 236-272 



HEAT. TREATMENT OF STEEL 



CHAPTER I 
HARDENING CARBON STEELS 

Originally the name steel was applied to various combinations 
of iron and carbon, there being present, together with these, as 
impurities, small proportions of silicon and manganese. At 
the present time, however, the use of the name is extended to 
cover combinations of iron with tungsten, vanadium, nickel, 
chromium, molybdenum, titanium and some of the rarer ele- 
ments. These latter combinations are quite generally known 
as the alloy steels to distinguish them from the carbon steels, in 
which latter the characteristic properties are dependent upon 
the presence of carbon alone. The alloy steels are divided into 
high-speed steels and low-carbon alloy steels. The specific prop- 
erties that distinguish these different steels are due in part to 
their respective compositions, that is, to the particular elements 
they contain, and, in part, to their subsequent working and 
heat-treatment. 

Effect of Difference in Composition of Steel. — In general, 
any change in the composition of a steel results in some change 
in its properties. For example, the addition of certain metallic 
elements to a carbon steel causes, in the alloy steel thus formed, 
a change in position of the proper hardening temperature point. 
Tungsten or manganese tend to lower this point, boron and vana- 
dium to raise it; the amount of the change is, generally, pro- 
portional to the amount of the element added. Just as a small 
proportion of carbon added to iron produces steel which has 
decidedly different properties from those found in pure iron, so 
increasing the proportion of carbon in the steel thus formed, 
within certain limits, causes a variation in the degree in which 



2 HEAT-TREATMENT OF STEEL 

these properties manifest themselves. For example, consider 
the property of tensile strength. In a " ten-point " carbon steel 
(one in which there is present but o.io per cent of carbon), the 
tensile strength is very nearly 25 per cent greater than in pure 
iron. Adding more carbon causes the tensile strength to rise, 
approximately, at the rate of 2.5 per cent for each 0.01 per cent 
of carbon added. 

Carbon steels are divided into three classes according to the 
proportion of carbon which they contain. The first of these 
embraces the " unsaturated " steels, in which the carbon con- 
tent is lower than 0.89 per cent; the second, the " saturated " 
steels, in which the proportion of carbon is exactly 0.89 per cent; 
and, the third, the " supersaturated " steels, in which the car- 
bon content is higher than 0.89 per cent. 

Effect of Heat-treatment. — With a steel of a given composi- 
tion, proper heat-treatments may be applied which, of them- 
selves, will first alter in form or degree some of its specific prop- 
erties, or second, practically eliminate one or more of these, or 
third, add certain new ones. Physical properties of size, shape 
and ductility are examples of the first case; an example of the 
second case is found in the heating of steel beyond its hardening 
temperature, which takes away its magnetism, making it non- 
magnetic; and an example of the third case is the fact that a 
greater degree of hardness may be added to steel by the process 
of hardening. In this connection it must be understood that, 
strictly speaking, hardness is a relative term and all steel has 
some hardness. 

There are three general heat- treatment operations: anneal- 
ing, hardening — with which this chapter will deal — and tem- 
pering. In all of these the object sought is to change in some 
manner the existing properties of the steel; in other words, to 
produce in it certain permanent conditions. 

The controlling factor in all heat- treatment is temperature. 
Whether the operation is annealing, hardening or tempering, 
there is for any certain steel and particular use thereof a definite 
temperature point that alone gives the best results. Insufficient 
temperatures do not produce the results sought. Excessive tern- 



HARDENING CARBON STEELS 3 

peratures, either through ignorance of what the correct point is 
or through inability to tell when it exists, cause " burned " steel; 
this is a common failing, resulting in great loss. Very slight 
variations from the proper temperature may do irreparable 
damage. 

Due to temperature variation alone, carbon steel may be had 
in any of three conditions: first, in the unhardened or annealed 
state, when not heated to temperatures above 1350 degrees F.; 
second, in the hardened state, by heating to temperatures be- 
tween 1350 and 1500 degrees F.; third, in a state softer than 
the second though harder than the first, when heated to temper- 
atures which exceed 1500 degrees F. 

The Hardening Process. — The hardening of a carbon steel is 
the result of a change of internal structure which takes place 
in the steel when heated properly to a correct temperature. In 
the different carbon steels this change, for practical purposes, is 
effective only in those in which the proportion of carbon is be- 
tween 0.20 per cent and 2.0 per cent, that is, between " twenty- 
point " and " two " carbon steels, respectively. 

When heated, ordinary carbon steels begin to soften at about 
390 degrees F. and continue to soften throughout a range of 
310 degrees F. At the point 700 degrees F. practically all of 
the hardness has disappeared. " Red hardness " in a steel is a 
property which enables it to remain hard at red heat. In a 
high-speed steel this property is of the first importance, 1020 
degrees F. being a minimum temperature at which softening 
may begin. This is some 630 degrees F. above the point at 
which softening commences in ordinary carbon steels. 

The process of hardening steel consists essentially of heating 
the steel to the required temperature and quenching it suddenly 
in some cooling medium. The methods of heating and the 
different kinds of quenching baths used will be explained in 
detail later. Generally speaking, the furnaces used for the 
heating of steel for hardening are heated either by gas, oil, 
electricity or solid fuel. Each of these methods has its advan- 
tages, according to the local conditions, the requirements on the 
work, the quantities of tools to be hardened, the cost of fuel, etc. 



4 HEAT-TREATMENT OF STEEL 

Electricity offers many attractive advantages for the heating 
of steel. The electric resistance furnace, as now built in a 
variety of sizes of either muffle or two-chamber types, has one 
fundamental advantage over coal, coke, gas or oil-heated fur- 
naces, which by many is claimed to render it quite superior. It 
is entirely free from all products of combustion, the heat being 
produced by electrical resistance. This is important, as it does 
away with the chief cause of oxidation of the heated steel. 
Further, the temperature of the electric furnaces can be easily 
and accurately regulated to, and maintained uniform at, any 
desired point. When electric power is generated for other pur- 
poses, the increased cost of this form of energy for operating 
furnaces is not sufficient to argue against it. Even when the 
current is purchased, the superior quality of work performed by 
this kind of furnace is claimed to frequently more than offset 
the slightly higher cost of operation. 

In the actual heating of a piece of steel, several requirements 
are essential to good hardening: first, that small projections or 
cutting edges are not heated more rapidly than is the body of 
the piece, that is, that all parts are heated at the same rate, and 
second, that all parts are heated to the same temperature. 
These conditions are facilitated by slow heating, especially when 
the heated piece is large. A uniform heat, as low in tempera- 
ture as will give the required hardness, produces the best prod- 
uct. Lack of uniformity in heating causes irregular grain and 
internal strains, and may even produce surface cracks. Any 
temperature above the " critical point " of steel tends to open 
its grain — to make it coarse and to diminish its strength — 
though such a temperature may not be sufficient to lessen 
appreciably its hardness. 

Critical Temperatures. — The temperatures at which take 
place the previously mentioned internal changes in the struc- 
ture of a steel are frequently spoken of as the " critical " points. 
These are different in steels of different carbon contents. The 
higher the percentage of carbon present, the lower the tempera- 
ture required to produce the internal change. In other words, 
the critical points of a high-carbon steel are lower than those of a 



HARDENING CARBON STEELS 5 

low-carbon steel. In steels of the commonly used carbon con- 
tents, there are two of these critical temperatures, called the 
decalescence point and the recalescence point, respectively. 

Decalescence and Its Relation to Hardening. — Everyone in- 
terested in the hardening of steel will have noticed the increasing 
frequency with which reference is made to the decalescence and 
recalescence points of steel, in articles appearing in the technical 
press from time to time. It is only during the past few years 
that this peculiarity in steel has come to the front, and there 
are still very many who do not possess even a rudimentary 
knowledge of the subject. The somewhat obscure references 
one usually sees in the treatises on hardening will not help the 
man in the hardening shop very much to a better understanding 
of the matter, and therefore an elementary explanation of the 
phenomenon will be welcome to many. It may be quoted that, 
as a matter of history, hardening has been done with more or 
less success, from the days of the famous Damascus swords up 
to only a comparatively short time ago, without anyone having 
discovered that steel possessed such a peculiarity as decales- 
cence, but nevertheless its relation to hardening has always 
existed, and its discovery paved the way for much scientific in- 
vestigation into a subject that had been previously controlled 
by rule of thumb. 

The " decalescence " and " recalescence " or " critical " points 
(also sometimes designated Ac. i and Ar. i), that bear relation to 
the hardening of steel, are simply evolutions that occur in the 
chemical composition of steel at certain temperatures during 
both heating and cooling. Steel at normal temperatures car- 
ries its carbon, which is its chief hardening component, in a 
certain form — pearlite carbon to be more explicit — and if 
heated to a certain temperature, a change occurs and the pearlite 
carbon becomes cementite or hardening carbon. Likewise, if 
allowed to cool slowly, the hardening carbon changes back again 
to pearlite. The points at which these evolutions occur are the 
decalescence and recalescence or critical points, and the effect 
of these molecular changes is to cause an increased absorption 
of heat on a rising temperature and an evolution of heat on a 



HEAT-TREATMENT OF STEEL 



falling temperature. That is to say, during the heating of a 
piece of steel a halt occurs, and it continues to absorb heat 
without appreciably rising in temperature, at the decalescence 
point, although its immediate surroundings may be hotter than 
the steel. Likewise, steel cooling slowly will, at a certain tem- 
perature, actually increase in temperature although its sur- 
roundings may be colder. This 
takes place at the recalescence 
point. 

In Fig. i is shown a curve, 
taken on a recording pyrom- 
eter, in which the decalescence 
and recalescence points are well 
developed. From this it will 
be seen that the absorption 
of heat occurred at a point 
marked 733 degrees C. (13 51 
degrees F.) on the rising tem- 
perature, and the evolution of 
heat at 724 degrees C. (1335 
degrees F.) on the falling tem- 
perature. The relation of these 
critical points to hardening is 
in the fact that unless a tem- 
perature sufficient to produce 
the first action is reached, so that the pearlite carbon will be 
changed to hardening carbon, and unless it is cooled with suf- 
ficient rapidity to practically eliminate the second action, no 
hardening can take place. The rate of cooling is material and 
accounts for the fact that large articles require to be quenched 
at higher temperatures than small ones. 

A very important feature is that steel containing hardening 
carbon, i.e., steel above the temperature of decalescence, is non- 
magnetic. Anyone may demonstrate this for himself by heating 
a piece of steel to a bright red and testing it with an ordinary 
magnet. While bright red it will be found to have no attraction 
for the magnet, but at about a cherry-red it regains its magnetic 









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Fig. 1. Curve made by a Recording 
Pyrometer showing the Decalescence 
and Recalescence Points 



HARDENING CARBON STEELS 7 

properties. This feature has been taken advantage of as a 
means of determining the correct hardening temperature, and 
appliances for its application are on the market. Its use is 
certainly to be recommended where no installation of pyrom- 
eters exists; the only point requiring judgment is the length 
of time an article should remain in the furnace after it has 
become non-magnetic. This varies with the weight and cooling 
surface, but may be tabulated according to weight, leaving very 
little to personal judgment. 

It is difficult to quote reliable temperatures at which decales- 
cence occurs. The temperatures vary with the amount of the 
carbon contained in the steel, and are much higher for high- 
speed than for ordinary crucible steel. Special electric furnaces 
are generally used for obtaining decalescence curves, but with 
care it can be done in an ordinary gas furnace, with a suitable 
pyrometer. All that is necessary is to bore a blind hole in a 
piece of the steel to be treated, to form a pocket to receive the 
end of the pyrometer. This must be of sufficient length to 
cover the resistance coil in the end of the pyrometer. The 
specimen should then be put into the furnace, with the pyrom- 
eter in, the gas applied, and, if the furnace is allowed to heat 
up very slowly toward a temperature of, say, 1380 degrees F. 
(750 degrees C), the decalescence curve will be developed, if 
the pyrometer is a recording one. In the same way, if the 
furnace is allowed to cool slowly it will be seen that at the 
recalescence point, the specimen gives off heat and even increases 
in temperature for a time. Experiments of this kind are scarcely 
practicable for the average hardening shop, but when it is de- 
sired to find the lowest hardening temperature for a piece of 
steel, the magnet can be used to advantage. 

Recapitulation. — To sum up, the decalescence point of any 
steel marks the correct hardening temperature of that particular 
steel. It occurs while the temperature of the steel is rising. 
The piece is ready to be removed from the source of heat directly 
after it has been heated uniformly to this temperature, for then 
the structural change necessary to produce hardness has been 
completed. Heating the piece slightly more may be desirable 



8 HEAT-TREATMENT OF STEEL 

for either or both of the two following reasons. First, in case 
the piece has been heated too quickly, that is, not uniformly, 
this excess temperature will assure the structural change being 
complete throughout the piece. Second, any slight loss of heat 
which may take place in transferring the piece from the furnace 
to the quenching bath may thus be allowed for, leaving the 
piece at the proper temperature when quenched. 

If a piece of steel which has been heated above its decales- 
cence point be allowed to cool slowly, it will pass through a 
structural change, the reverse of that which takes place on a 
rising temperature. The point at which this takes place is the 
recalescence point and is lower than the rising critical tempera- 
ture by some 85 to 215 degrees F. The location of these points 
is made evident by the fact that while passing through them the 
temperature of the steel remains stationary for an appreciable 
length of time. It is well to observe that the lower of these 
points does not manifest itself unless the higher one has been first 
fully passed. As these critical points are different for different 
steels, they cannot be definitely known for any particular steel 
without an actual determination. While heating a piece of steel 
to its correct hardening temperature produces a change in its 
structure which makes possible an increase in its hardness, this 
condition is only temporary unless the piece is quenched. 

Quenching. — The quenching consists in plunging the heated 
steel into a bath, cooling it quickly. By this operation the struc- 
tural change seems to be " trapped " and permanently set. 
Were it possible to make this cooling instantaneous and uniform 
throughout the piece, it would be perfectly and symmetrically 
hardened. This condition cannot, however, be realized, as the 
rate of cooling is affected both by the size and shape of the treated 
piece; the bulkier the piece, the larger the amount of heat that 
must be transferred to the surface and there dissipated through 
the cooling bath; the smaller the exposed surface in comparison 
with the bulk, the longer will be the time required for cooling. 
Remembering that the cooling should be as quickly accom- 
plished as possible, the bath should be amply large to dissipate 
the heat rapidly and uniformly. Too small a quenching bath 



HARDENING CARBON STEELS 9 

will cause much loss, due to the resulting irregular and slow 
cooling. To insure uniformly quenched products, the tempera- 
ture of the bath should be kept constant, so that successive 
pieces immersed in it will be acted upon by the same quenching 
temperature. Running water is a satisfactory means of pro- 
ducing this condition. 

The composition of the quenching bath may vary for different 
purposes, water, oil or brine being used. Greater hardness is 
obtained from quenching, at the same temperature, in salt 
brine and less in oil, than is obtained by quenching in water. 
This is due to a difference in the heat-dissipating power pos- 
sessed by these substances. Quenching thin and complicated 
pieces in salt brine is unsafe as there is danger of the piece 
cracking, due to the extreme suddenness of cooling thus pro- 
duced. 

In actual shop work the steel to be hardened is generally of a 
variety of sizes, shapes and compositions. To obtain uniformity 
both of heating and of cooling, as well as the correct limiting 
temperature, the peculiarities of each piece must be given con- 
sideration in accordance with the points outlined above. In 
other words, to harden all pieces in a manner best adapted to 
but one piece would result in inferior quality and possible loss 
of all except this one. Each different piece must be treated 
individually in a way calculated to bring out the best results 
from it. 

Theory of Critical Points. — The presence of the critical points 
in the heating and cooling of a piece of steel is a phenomenon. 
The most reasonable explanation is as follows : 

While heating, the steel uniformly absorbs heat. Up to the 
decalescence point all of the energy of this heat is exerted in 
raising the temperature of the piece. At this point, the heat 
taken on by the steel is expended, not in raising the tempera- 
ture of the piece, but in work which produces the internal 
changes here taking place between the carbon and the iron. 
Hence, when the heat added is used in this manner, the tempera- 
ture of the piece, having nothing to increase it, remains station- 
ary, or, owing to surface radiation, may even fall slightly. 



10 HEAT-TREATMENT OF STEEL 

After the change is complete, the added heat is again expended 
in raising the temperature of the piece, which increases propor- 
tionally. 

When the piece has been heated above the decalescence point 
and allowed to cool slowly, the process is reversed. Heat is 
then radiated from the piece. Until the recalescence point is 
reached, the temperature falls uniformly. Here the internal 
relation of the carbon and iron is transformed to its original con- 
dition, the energy previously absorbed being converted into heat. 
This heat, set free in the steel, supplies, for the moment, the 
equivalent of that being radiated from the surface, and the tem- 
perature of the piece ceases falling and remains stationary. 
Should the heat resulting from the internal changes be greater 
than that of surface radiation, the resulting temperature of the 
piece will not only cease falling but will obviously rise slightly 
at this point. In either event the condition exists only momen- 
tarily, but when the carbon and iron constituents have resumed 
their original relation, the internal heating ceases, and the tem- 
perature of the piece falls steadily, due to surface radiation. 

Apparatus for Determining the Critical Points. — From the 
foregoing sections it is evident, first, that there is a definite 
temperature at which any carbon steel should be hardened, and, 
second, that a great loss occurs, both of labor and material, 
unless the hardening is carried out at this temperature. The 
actual shop problem thus presented is to determine readily and 
accurately the correct hardening temperature for any carbon 
steel that may be in use. This can be done by the use of various 
types of pyrometers; an apparatus made by the Hoskins Mfg. 
Co., of Detroit, Mich., is well adapted for the purpose. This 
apparatus consists of a small electric furnace in which to heat a 
specimen of the steel to be tested, and a special thermo-couple 
pyrometer for indicating the temperature of this specimen 
throughout its range of heating. The specimen itself should be 
properly shaped for clamping to the thermo-couple. 

The furnace may be operated on either alternating- or direct- 
current circuits. The furnace chamber is 2-jg- inches in diam- 
eter and 2 \ inches deep. Heat is produced by means of the 



HARDENING CARBON STEELS II 

resistance offered to the passage of an electric current through 
the " resistor " or heating element which in the form of wire is 
wound in close contact with the chamber lining. The furnace is 
designed so that it can be used on standard lighting circuits to 
which ready connection is made with a twin conductor cord and 
lamp plug. In operation, it consumes 3I amperes at no volts, 
and is capable of producing a chamber temperature of 1830 
degrees F., which is considerably higher than required for a 
carbon steel. 

The pyrometer consists of a thermo-couple, connecting leads 
and indicating meter. The thermo-couple is of small wire so 
as to respond quickly to any slight variation in temperature. 
The welded end of this couple is slightly flattened to enable a 
good contact between it and the steel specimen. The meter is 
portable and indicates temperatures up to 2552 degrees F. 

The specimen of the steel to be tested should be small, so as 
to heat quickly and uniformly. A well-formed specimen is made 
with two duplicate parts, each ij inch long by J inch wide by 
J inch thick. The pieces are clamped by means of two f-inch 
bolts, one on each side of the welded part of the extreme end 
of the thermo-couple. Care is taken to form a tight contact, 
though not to cause an undue strain on the couple. The dimen- 
sions here given for the test specimen are not essential, though 
convenient; any pieces which will permit of tight contact with 
the thermo-couple and of heating in the furnace chamber may 
be used. 

With the specimen fastened to the couple as just described 
the furnace is connected in circuit and the cover placed over the 
chamber opening. The temperature within the chamber rises 
steadily. When it becomes 1 700 degrees F., the end of the couple, 
with specimen attached, is inserted in the chamber. The steel 
specimen rapidly heats, its temperature being constantly the 
same as that of the welded junction of the thermo-couple, due 
to the intimate contact between them. This temperature, in- 
dicated by the meter, will rise uniformly until the decalescence 
point of the steel tested is reached. At this temperature the 
indicating needle of the meter becomes stationary, the added 



12 HEAT-TREATMENT OF STEEL 

heat being consumed by internal changes. These changes com- 
pleted, the temperature again rises, the length of the elapsed 
period of time depending upon the speed of heating. With the 
furnace temperature kept nearly constant at the initial point, 
here given as 1700 degrees F., this " speed of heating " will be 
such as to allow of readily observing the pause in motion of the 
needle. The temperature at which this occurs should be care- 
fully noted. 

To obtain the lower critical point, the temperature of the 
piece is first raised above the decalescence point by about 105 
degrees F. In this condition it is removed from the furnace 
and rested on top to cool. The decrease of temperature is at 
once noticeable by the fall of the meter needle. At a tempera- 
ture somewhat below the decalescence point, varying with the 
composition of the steel, as previously mentioned, there is again 
a noticeable lag in the movement of the needle. The tempera- 
ture at which the movement ceases entirely is the recalescence 
point. Immediately following there may occur a slight rising 
movement of the needle, as previously explained. 

During these intervals of temperature lag, both during the 
heating and cooling of the steel, there may occur a small fluctu- 
ation in the temperature. In order to get results that are com- 
parable, a definite point in each of these intervals should be con- 
sidered each time a test is made. Hence, both the decalescence 
and recalescence temperatures are taken as the points at which 
the needle first becomes stationary. As all operations of heat- 
treatment of a steel center around its critical points, the im- 
portance of knowing these exactly is realized; to make certain, 
each test should be checked by a second reading. The time re- 
quired for this is small. A close agreement of two succeeding read- 
ings will give assurance of the correctness of the determination. 

Results Obtained from Sample Specimens. — In order to 
show graphically the necessity of quenching carbon steels at the 
proper temperature points, a series of specimen pieces of the 
same steel were treated at different temperatures. The steel 
used contained exactly 1 per cent carbon. A number of test 
specimens were made of this from adjacent parts of the same bar. 



HARDENING CARBON STEELS 



J 3 



First the critical points of this steel were determined. Tem- 
peratures were recorded throughout both the heating and cool- 
ing. In the diagram, Fig. 2, these values have been plotted. 
The curve shows graphically the location of the critical points, 
and also the slight fall or rise of temperature as the case may be. 

With this data obtained, seven specimens of the same steel 
were heated in the electric furnace, each to a different tempera- 
ture. As these pieces were removed from the furnace they were 
immediately quenched in water. The temperature of the 









TEMPERATURE (DEGREES F.) 

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IMEN RE 


MOVED F 


ROM FUF 


NACE 










DECALE 


SCENT t 


OINT 

V 


A 




















r 


\ 


















/ 


1 




\ / R 


ECALESC 


■NT POU 


IT 










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)123456789 10 1 

TIME (MINUTES) 

Machine' 


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Fig. 2. Diagram showing the Relation between Time and Temperature 
when Heating Steel, and the Critical Temperatures of One-per-cent 
Carbon Steel 

quenching bath was held constant at 45 degrees F. The hard- 
ened pieces were then broken at right angles and the fractured 
surface of each was photographed imder a microscope. An in- 
spection of the photographs at once showed the serious effects 
of overheating on the structure of the steel and hence on its 
strength. 

One specimen was hardened just as the temperature reached 
the decalescence point. This showed clearly the direction in 
which the hardening takes place, namely, from the exterior 



14 HEAT-TREATMENT OF STEEL 

toward the interior. This would naturally be expected as the 
temperature of the surface, which is exposed directly to the 
source of heat, reaches the critical point first. This condition 
indicates the necessity of heating the piece uniformly. 

Tempering. — Although the mistake is rarely made in the 
hardening-room itself, it is surprising how often engineers speak 
of " tempering " when they mean " hardening." The process 
of hardening consists of raising the steel to a proper red heat, 
and then cooling it rapidly by quenching in water, or by other 
means. The tempering is a further process of heating the 
steel to a much lower temperature than for hardening, and the 
effect is to toughen the steel, and also, unfortunately, to soften 
it somewhat. The first process is sometimes erroneously called 
tempering, and more frequently this word is applied in general 
terms to the two processes together, but it is entirely wrong to 
do this. The distinction is made not simply by a purist in the 
use of correct language, but because the processes are so dis- 
tinct that it is very frequently necessary to distinguish between 
them in order to avoid confusion. 

There are many other terms and expressions in the hardening- 
room which are loosely used, and in some cases confusion of 
thought results. Some expressions which are incorrect in them- 
selves have so passed into current use that they must, perhaps, 
be tolerated and made the best of. For example, when a man 
says that his tools are " too hard " he almost always means that 
they are too brittle. Surely the hardness is a good thing, and 
for most purposes a tool cannot be too hard. Even a punch, 
which under existing circumstances ought to be tempered until 
it can just " be touched by a file," would be much better if it 
could be made glass-hard and at the same time so tough that it 
would not break or chip. As a matter of fact, the punch must 
be tempered until it loses some of its hardness, not because it is 
too hard, but because it is too brittle. It would be better if the 
brittleness could be removed without impairing the hardness. 
The loose expression has established itself because the ideas of 
hardness and brittleness are so intimately associated in men's 
minds that frequently they can hardly be dissociated. There 



HARDENING CARBON STEELS 15 

are times, however, when the expression which is so frequently 
misused is really required in its correct sense; for example one 
sometimes uses a lead or copper hammer instead of a steel one, 
because the steel hammer would be too hard and would bruise 
the work. 

The Use of Pyrometers. — The hardening of carbon steels for 
highest quality and greatest saving entails, then, three things. 
First, a definite knowledge of what constitutes the correct 
temperature at which to harden the steel. The second point 
necessitates a positive means of accurately determining this 
hardening temperature for any carbon steel. The third considera- 
tion is that the correct hardening temperature, once determined, 
is actually carried out in the hardening work. A simple and 
effective way of doing this is by checking the temperature of 
the hardening furnace by means of a pyrometer. When there 
is a large quantity of work to be hardened, economy dictates a 
permanent installation of pyrometers. The convenience of such 
installations is manifest. A thermo-couple is placed in each 
furnace. A number of these, from three to sixteen, depending 
upon individual conditions, are connected by wire leads, through 
a selective switch to one meter. By a turn of the switch, the 
temperature of any furnace may be read at once from the meter. 
This makes it possible for the foreman to know definitely at a 
single point, the temperatures of all of the hardening furnaces 
in use. 

The Brayshaw Experiments on the Heat-treatment of Steel. 
— Having now reviewed the principles underlying the harden- 
ing of steel, we are ready to take up in greater detail the 
processes and devices used for obtaining the required results. 
Before doing so, however, a brief account will be given of a remark- 
able series of experiments that have been made to ascertain, as 
definitely as possible, the laws that govern the heat-treatment 
of steel. 

The hardening of steel has always been considered an opera- 
tion for which definite rules could not be laid down, but in which 
the experience and judgment of the hardener would almost ex- 
clusively have to be relied upon. Practically the only definite 



1 6 HEAT-TREATMENT OF STEEL 

rules that have been laid down are that steel should be hardened 
at as low a heat as possible, and that there is a definite tempera- 
ture for each kind of steel above which it must be heated to 
harden at all. 

Apart from this meager information, a few generalities only 
have been furnished for the guidance of men doing this work, 
including directions for cooling, so as to minimize the risk of 
cracking; but definite information on the process of hardening 
is singularly lacking, and many have considered it impossible 
to shape rules for this operation, even as the result of careful 
experiments. 

For this reason the experiments conducted by Mr. Shipley N. 
Brayshaw of Manchester, England, the results of which were 
reported in a paper read before the Institution of Mechanical 
Engineers at the April 15, 1910, meeting, and which are recorded 
in detail in the following, are all the more remarkable and of 
great interest to everyone engaged in mechanical work. These 
experiments appear to have been made with extraordinary care, 
and two of the points brought out in his investigations deserve 
to be particularly mentioned. One of them relates to the short- 
ening or lengthening of steel in hardening. It has often been 
stated that steel is unreliable and not uniform in this respect, 
and that the same kind of steel has sometimes been known to 
shorten in hardening, and sometimes to lengthen. This phe- 
nomenon has been attributed to defects, or, at least, to special 
conditions in the steel, over which the hardener has no control. 
Mr. Brayshaw's experiments, however, show that the shorten- 
ing and lengthening of steel in hardening follows a uniform law, 
and that steel hardened below a given temperature, which he 
calls the " change-point," will shorten when quenched; whereas 
the same steel, if heated above this change-point before quench- 
ing, will lengthen. He also shows that by heating the steel in 
two furnaces, first bringing it up to a certain temperature in 
one, and then soaking it for a given time at a definite tempera- 
ture in the other, it is possible to harden steel so that it will 
neither lengthen nor shorten when quenched. This is, without 
doubt, the first definite information that has been recorded on 



HARDENING CARBON STEELS 17 

the change in the length of steel in hardening, the uncertainty 
of which has caused considerable difficulty in making accurate 
taps, dies, etc. 

Another interesting point brought forth by Mr. Brayshaw's 
experiments relates to the proper hardening temperature. While 
it is possible to harden steel within a temperature range of 
about 200 degrees F., and obtain what to the ordinary observer 
would seem to be good results, the best results are always ob- 
tained within a very narrow range of temperatures, approaching 
closely the decalescence point, or the temperature at which 
steel changes into a condition when it can be hardened by 
quenching. It is interesting to note that this result agrees 
with the old theory that steel should be hardened at as low a 
temperature as possible. In a number of cases certain tools are 
found to last exceptionally well, while other tools in the same 
lot show only ordinary durability, although no difference can be 
detected in the grain of the hardened steel. Such differences 
can now be accounted for by the slight variations in the tem- 
perature at which the various tools have been hardened. 

These experiments open up an entirely new field for investi- 
gation and new possibilities in the hardening of tool steel. They 
indicate that in cases where it is important to prevent the 
cracking of tools in hardening and the deformation of tools due 
to internal stresses, they should be hardened at a temperature 
higher than thatf which gives the best results as regards hardness 
only. Thus we find that certain of the desirable qualities in a 
hardened tool are antagonistic; that is, we are unlikely to obtain 
a tool having extreme hardness and elastic limit and which at 
the same time is not likely to crack or lose its shape. Under 
these conditions one of the necessary qualities must be partly 
sacrificed to another, and a tool must be hardened so as to 
obtain good general results rather than the best specific one. 

The influence of previous annealing on hardening is also of 
interest; and many of the points brought out by the results of 
these experiments are well worth considering. The experiments 
deal exclusively with the results obtained from two kinds of 
carbon tool steel that, except for minute variations, differed 



18 HEAT-TREATMENT OF STEEL 

only in the fact that one of them contained about 0.5 per cent of 
tungsten. The steel contained on an average of 1.16 per cent 
carbon, 0.15 per cent silicon, 0.36 per cent manganese, 0.018 per 
cent sulphur, and 0.013 per cent phosphorus. The whole work 
of investigation was devoted to questions directly connected 
with machine shop hardening, with the aim in view of throwing 
light on the many problems met with in daily practice. 

Hardening Temperatures. — The hardening point of both low- 
tungsten and carbon steel may be located with great accuracy, 
and the complete change from soft to hard is accomplished within 
a range of about 10 degrees F., or less. After the temperature 
has been raised more than from 35 to 55 degrees F. above the 
hardening point, the hardness of the steel is lessened by further 
increases in the temperature, provided the heating is sufficiently 
prolonged for the steel to acquire thoroughly the condition 
pertaining to the temperature. There is a " change-point " at 
about 16 1 5 degrees F. in low- tungsten steel and at a somewhat 
higher temperature in carbon steel. One of the several indica- 
tions of this change-point is the shortening of bars hardened in 
water at temperatures below that point, whereas the bar length- 
ens if this temperature is exceeded at the time of quenching. 
Practically the same results are obtained by heating low-tung- 
sten bars to any temperature from 1400 to 1725 degrees F. and 
quenching in oil, as by quenching in water. 

Length of Time of Heating. — Regarding the effect of heating 
to various temperatures for various lengths of time before 
quenching for hardening, the following conclusions are drawn: 
Prolonged soaking up to 120 minutes at temperatures at which 
the hardening change is half accomplished in 30 minutes, does 
not suffice to complete the change. Prolonged soaking for 
hardening at a temperature of 1400 degrees F. has a slightly 
injurious effect on the steel, but does not materially influence 
the hardness. At a temperature of about 1490 degrees F. a 
great degree of hardness is attained by quick heating, but the 
hardness is impaired with 30 minutes soaking. Prolonged soak- 
ing for hardening at a temperature of about 161 5 degrees F. has 
a seriously injurious effect upon the steel. A specially great 



HARDENING CARBON STEELS 19 

degree of hardness may be obtained by means of soaking at a 
high temperature, as at 16 15 degrees F. for a very short time, 
but even as long a time as 7J minutes is long enough to seriously 
impair the hardness. 

The temperature of brine for quenching is of considerable 
importance. Both low- tungsten and carbon steel bars quenched 
at 41 degrees F. were decidedly harder than bars quenched at 
75 degrees, and quenching at 124 degrees F. rendered the bars 
much softer. 

Effects of Previous Annealing. — The method of previous an- 
nealing affects the hardness of steel considerably. The elastic 
limit of low- tungsten bars hardened at either 1400 or 1580 de- 
grees F. varies according to the annealing they have undergone. 
The elastic limit is high after annealing at about 1470 degrees F. 
for 30 minutes, or 1290 degrees F. for 120 minutes, but it is 
seriously impaired by annealing at 1470 degrees F. for 120 min- 
utes. If low- tungsten steel is annealed at 1725 degrees F. and 
hardened at 1400 degrees F., the elastic limit is inferior, and the 
adverse effect of the previous annealing is much more pronounced 
if the hardening is done at 1580 degrees F. The elastic limit of 
carbon steel annealed at any temperature between 1290 and 1725 
degrees F. and hardened at either 1400 or 1580 degrees F. does 
not vary by nearly such great amounts as the elastic limit of the 
low-tungsten bars, and the highest annealing temperature given 
above is not injurious so far as the elastic limit is concerned. 

The hardness of low- tungsten bars hardened at 1400 degrees F. 
decreases from a high scleroscope figure to a low one as the tem- 
perature of annealing increases from 1290 to 1725 degrees F. 
The hardness is increased by prolonging the annealing at the 
lower temperature. The hardness of low- tungsten steel hard- 
ened at 1580 degrees F. is fairly constant at a moderately high 
sceleroscope figure, whatever the temperature of annealing. 

Effect of Heating in Two Furnaces. — An interesting part 
of the experiments relates to the use of two furnace heats for 
hardening, heating the steel first in one furnace to a certain tem- 
perature for a given time, and then immediately, without cool- 
ing, soaking in a second furnace at a known temperature and 



20 HEAT-TREATMENT OF STEEL 

for a definite time. These experiments show that low- tungsten 
and carbon steel bars heated for half an hour to temperatures 
between 1545 and 1650 degrees F. are not much affected so far 
as their elastic limit and maximum strength are concerned by a 
further immediate soaking for half an hour at 1400 degrees F. 
If, however, the temperature in the first furnace is 1725 degrees F., 
the low-tungsten steel is much improved by a further soaking 
at 1400 degrees F., but the carbon steel is much injured by the 
same treatment. Bars of low-tungsten steel heated for 30 min- 
utes, at 161 5 degrees F. and then soaked at 1332 degrees F. for 
a further 30 minutes, give a high elastic limit and maximum 
strength, and are harder than if the second soaking were at a 
temperature of 1400 degrees F. The carbon steel, again, is but 
little affected by these variations in the second furnace. 

The change of length in hardening, however, of both low- 
tungsten and carbon steel is much affected by the above varia- 
tions in the temperature of the second furnace. Good results 
as regards elastic limit and maximum strength, and also as 
regards hardness, are obtained by very short soaking, first at a 
high temperature, say 161 5 degrees F., and then at a low one, 
the results being best when the second temperature is near to 
or a little below the hardening point. If the furnace be at a 
sufficiently high temperature it is easy either by variations of 
the temperatures of the two furnaces, or by variations in the 
time of soaking, to arrive at a treatment of the steel, both low- 
tungsten and carbon, whereby they neither lengthen nor shorten. 
Under the same treatment carbon steel has a greater tendency 
to shorten than low-tungsten steel. 

Miscellaneous Results. — Other experiments showed that low- 
tungsten steel heated to 1580 degrees F. for 15 minutes and 
quenched in oil has a higher elastic limit and is harder than 
carbon steel similarly treated. As to annealing, it was found 
that bars annealed at a temperature of 1470 degrees F. or below 
became slightly shorter by the annealing process, and this action 
was more pronounced in the case of carbon steel than tungsten 
steel. Annealing at a temperature of 1650 degrees F. causes 
both low-tungsten and carbon steel to lengthen. 



HARDENING CARBON STEELS 21 

It was found that recalescence of low-tungsten steel takes place 
gradually at a temperature of 1348 degrees F., and more readily 
at 1337 degrees F., and further that the recalescence at either 
of the above temperatures is very much retarded if the steel is 
cooled from a maximum heat of 1634 degrees F. 

Regarding hardening cracks, it is shown that both for low- 
tungsten and carbon steel, such treatment as produced the 
highest elastic limit accompanied by the greatest hardness is 
frequently the most risky. The risk of hardening cracks is re- 
duced if the steel is heated for a sufficient length of time to a 
temperature of 1650 degrees F. or a little above. Low- tungsten 
steel is more liable to crack in hardening than is carbon steel. 

Effect of Tempering. — Tempering experiments showed that 
little effect was produced by the tempering of carbon steel to 
300 degrees F. for 30 minutes. Tempering the same steel to 
480 degrees F. for 15 minutes, however, caused it to soften con- 
siderably and to shorten in length. For low-tungsten steel the 
elastic limit was increased considerably by tempering up to a 
temperature of 480 degrees F. The maximum strength of the 
same steel coincides with the elastic limit for bars either un- 
tempered or tempered at 300 degrees F. for 15 minutes, but it 
then rises rapidly with further tempering. The hardness, as 
measured by the scleroscope, was considerably reduced by tem- 
pering at 300 degrees F. and still more at 390 degrees F., but 
was not so much affected by further tempering at 480 degrees F. 
The length of the low-tungsten bars was reduced by tempering 
up to a temperature of 480 degrees F.; the higher the tempera- 
ture, the greater was the reduction in length. 

Effect on Tensile Strength. — The following conclusions refer 
to low- tungsten steel, but there is no reason to doubt that they 
are also applicable to carbon steel. A variation in the harden- 
ing temperature of only 9 degrees F., the extremes being re- 
spectively above and below the proper hardening temperature 
or decalescence point, had a tremendous influence on the exten- 
sion under load, but the maximum strength of the bars so 
treated did not differ much. A very good bar was produced by 
quenching from a temperature fully 108 degrees F. above the 



22 HEAT-TREATMENT OF STEEL 

hardening temperature. A heat of only 5 minutes' duration 
produced a harder bar than a heat of 25 minutes, the maximum 
temperature in both cases being 1470 degrees F., or a little above; 
but the bar heated for a shorter time gave a much lower elastic 
limit. The maximum strength alone is not necessarily any in- 
dication of the condition of the steel in question, or of the treat- 
ment to which it has been subjected; nor is the hardness alone 
necessarily an indication of the condition of the steel or the 
treatment. 

The following conclusions refer both to tungsten and carbon 
steels. Tempering up to a temperature of 570 degrees F. grad- 
ually increases the maximum strength and the elastic limit, 
although some irregularities enter which have not been fully 
accounted for. Tempering to this temperature reduces, for 
a given stress, the extension under load and the permanent 
extension. 

In conclusion it may be stated that these experiments show 
that steel of the quality treated in these experiments may be 
hardened within a temperature range of about 215 degrees F. 
The lower end of this range is very sharply defined, but the 
highest temperature allowable is difficult to determine, and as 
far as the appearance of the fracture is concerned there is but 
little evidence of improper hardening until the temperature of 
the proper hardening point has been exceeded by 270 degrees F. 
So wide, in fact, is the margin of allowable variation for harden- 
ing that when the hardness is decided by the appearance of the 
fracture alone, any workman of average skill can easily keep 
within the limits and judge the temperature by sight alone, 
and as a matter of fact this is being done all the time in the man- 
ufacture of such articles as pocket knives, small files, etc., which 
are hardened by the thousands with practically no waste. But, 
of course, it must not be understood that articles so hardened 
reach anything like their maximum efficiency, because even 
small variations in the heat-treatment previous to the quenching 
have a pronounced effect upon the condition of the steel, and 
even the previous treatment, such as the annealing to which the 
steel has been subjected, may influence the final result. 



HARDENING CARBON STEELS 23 

While it is thus easy to harden so as to obtain reasonably 
good results, the production of the best results necessitates a 
high degree of accuracy which can never be obtained by sight 
alone, and it is also important to notice that the difference 
between good hardening and the best hardening is very great. 
As an example may be mentioned the hardening of razors. It is 
sometimes said that whatever price one pays for a razor, the 
buying is a game of chance. Occasionally one hears of a re- 
markable razor that holds its edge as if by magic, while others 
of the same make and type may not be anywhere near as good. 
All of them, however, would show to the eye practically the 
same fracture, and apparently seem to have been treated in the 
same way. The experiments referred to above, however, indi- 
cate that there may have been a slight difference in the harden- 
ing temperature and consequently in the subsequent condition 
of the steel, and also that it would be possible to harden every 
razor in a gross so that each one would be truly a duplicate of 
the best. The same, of course, holds true of a great number of 
other tools. 

There have, for some years, been efforts made by steel makers 
to discover new alloy steels, and splendid succcess has been ob- 
tained in this direction, but there is still a wide field for the steel 
users for discovering the best use of the material already known. 
It is of little avail that occasionally tools show marvelous results, 
unless the hardener can at any time produce the same results 
with the same steel. The time is likely to come when all the 
factors in the hardening of tool steel will be controlled with 
accuracy within predetermined limits, and any failure may be 
investigated and the cause located with as much certainty as 
if a mistake had been made in the machine shop. 

Carbon Steel vs. High-speed Steel. — The idea that the 
proper hardening of carbon steel will make it possible to obtain 
better results by the use of this steel than has ordinarily been 
the case in the past has been expressed in this country as well 
as in England. At least one large machine tool building and 
tool-making firm has made extensive inquiries in the direction 
of determining proper methods for hardening ordinary carbon 



24 HEAT-TREATMENT OF STEEL 

steel so as to obtain the best results, and several writers on me- 
chanical subjects who have investigated the subject concur in 
the opinion that carbon steel, properly heat-treated, can be 
made to produce better results than is usually expected. One 
writer in Machinery gave voice to this opinion as follows: 

" High-speed steel is in fashion nowadays. This fact to- 
gether with the high degree of skill required to get the best 
results from carbon steel has caused this steel to be neglected. 
For some kinds of work, however, carbon steel is superior to 
any high-speed steel on the market, if it is dressed properly and 
receives the proper heat-treatment. 

" The carbon in steel may be in one of two forms: Annealed 
steel has the carbon in the non-hardening or cementite form. 
Hardened steel has the carbon in the hardening or martensite 
form. That these are two distinct forms may be seen by taking 
a small piece of annealed steel and a small piece of hardened steel 
and dissolving each in hydrochloric acid. The annealed steel 
will dissolve, leaving a black residue, while the hardened piece 
dissolves, leaving no residue. This shows that in one case some 
of the carbon is in the free or graphitic form and does not dis- 
solve in acid, while in the other case, the carbon is in the com- 
bined form and all dissolves." 

Regarding the use of the magnetic needle for indicating the 
correct hardening temperature, the same writer says: 

" There are two hardening methods which depend on the fact 
that steel loses its magnetic properties when the hardening point 
is reached. A piece of steel heated to a dull red and brought 
into the plane of a magnetic needle will attract the needle. If 
heated until the temperature is above the hardening point, there 
will be no attraction and the needle will not be affected by the 
presence of the steel. In using this test, care must be taken 
that the presence of the tongs does not mislead the workman. 
The comparatively cool tongs may attract the needle even when 
the steel is above the critical point. An ordinary horse-shoe 
magnet may be used instead of the magnetic needle. It is less 
likely to mislead because of the tongs or cooler parts of the steel, 
but is less sensitive. A bar magnet hung on a pivot at the center 



HARDENING CARBON STEELS 25 

and provided with a handle can be used very satisfactorily. It 
can be introduced into the furnace to test the steel during the 
process of heating and is more convenient than either of the 
other methods. It is not necessary to test every piece of steel, 
but a test should be made whenever the person takes another 
grade of steel or whenever the light changes. The intensity of 
the light makes a great difference in the color of a piece of iron 
or steel at a given temperature." 

General Rules for Hardening. — If steel workers would ob- 
serve the following in all cases when hardening steel, they would 
have better results: Harden carbon steel at the lowest possible 
heat and always on the rising heat. The last part of this rule is 
the one more often overlooked. Steel may be forged at a higher 
heat than the hardening heat, but should in all cases be annealed 
before being heated for hardening. The grain of the steel cor- 
responds to the highest heat it has received since it was black. 
If a piece of steel is forged at 1600 degrees F. and allowed to 
cool down to 1400 degrees F. to harden, it will have a grain 
corresponding to 1600 degrees F. 



CHAPTER II 
HEATING THE STEEL FOR HARDENING 

The furnaces used in hardening and tempering are heated 
either by gas, oil, electricity or solid fuels, such as coal and 
coke. Furnaces using oil or gas are made in many different 
styles and sizes to suit various classes of work, but differ very 
little in their general arrangement. 

Simple Type Gas Furnace. — Fig. i shows at B and C a very 
simple type of crucible furnace in which gas is burned under 
moderate air blast. There should be not less than two burners 
when this arrangement is used. The section B shows the ar- 
rangement of these burners in the vertical plane and the section 
C, their arrangement in the horizontal plane. With this arrange- 
ment of burners, the flame will travel several times around the 
crucible instead of passing as nearly as possible in a straight 
line from the burner to the vent, as would be the case if the 
furnace were constructed as at A, which is an objectionable 
design. 

At D, in the same illustration, is shown a section of a burner 
which can be made up from standard pipe fittings. If the tip 
burns out, as it probably will in time, the flanged joint enables 
it to be easily removed, and the burned parts can be cheaply 
and quickly replaced. Between the flanges, a gasket of asbes- 
tos paper or similar material should be used. For a furnace of 
the dimensions indicated at A, fittings for i|-inch wrought-iron 
pipe will be large enough, if two burners are used, and a 2 -inch 
pipe will be sufficient to bring the gas and air from the mixer to 
the tee supplying the burners. If the air used carries a pressure 
of forty pounds per square inch or more, the air should, for the 
sake of economy, be used in a jet blower. A design of a com- 
bined jet blower and mixer which can easily be designed from 
pipe and fittings is shown in Fig. 2. The fittings should not be 

26 " 



HEATING FURNACES 



27 



smaller than 2-inch pipe size for the cross, ij-inch pipe for the 
gas, and |- or ^-inch pipe for the compressed air. The pressure 
beyond the mixer need not be more than two pounds per square 
inch, and often one pound is sufficient. 

Gas furnaces use either natural, artificial or producer gas. 
Some gas furnaces are equipped with an automatic apparatus 
which operates in conjunction with a pyrometer for controlling 
the temperature to within a few degrees of a given point. The 



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Simplest Type of Crucible Gas Furnace showing Objectionable 
and Approved Designs 



air supply is generally obtained from a positive blower, although 
when a compressor is installed for operating pneumatic tools, 
the air is sometimes utilized for the furnaces by interposing 
reducing valves to diminish the pressure. Artificial gas is more 
expensive than oil, but is cleaner, and the installation of supply 
tanks, such as are required for oil, is avoided. Producer gas 
obtained from a separate plant is not economical unless there is 
a considerable number of furnaces. When oxidation or the for- 
mation of scale is particularly objectionable, furnaces of the muffle 



28 



HEAT-TREATMENT OF STEEL 



type are often used, having a refractory retort in which the steel 
is placed so as to exclude the products of combustion. These 
muffles must be replaced very frequently and more fuel is re- 
quired than when an oven type of furnace is used. 

Oil-burning Furnaces. — The use of oil in furnaces for the 
heat-treatment of steel possesses, in many cases, certain advan- 
tages over other methods of heating. Chief among these ad- 
vantages is the consideration of economy, as oil in the past, at 
least, has generally been cheaper to use than any other available 
form of fuel. The consideration of economy is limited, how- 









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Machinery 



Fig. 2. Combined Jet Blower and Mixer made from Pipe Fittings 

ever, by a somewhat increased complication in the method of 
operation, and on this account, oil is not recommended for fur- 
naces that will operate on gas with a consumption of 230 cubic 
feet per hour or less. The best results with oil-heated furnaces 
are secured with the larger-sized units. 

Before entering upon a description of any specific type of 
furnace, it will doubtless be advisable to give a brief description 
of the method on which oil furnaces operate. The oil is fed to 
the furnace from a tank underground, a pumping system being 
used to maintain a pressure of about 40 pounds per square inch 
in the supply pipe. The oil emerges from the burner through 
an orifice about yg inch in diameter. This orifice is surrounded 
by a second pipe through which steam or air is supplied under 



HEATING FURNACES 29 

pressure. The fine stream of oil is taken up by this stream of 
compressed air or steam and " atomized " or broken up into 
very finely divided particles. The oil mixed with the air in 
this way forms the combustible mixture that is burned in the 
furnace. 

In those cases where the oil is atomized by steam, it is neces- 
sary to supply a certain amount of additional air to get the 
desired combustion. To understand the way in which this com- 
bustion proceeds, it must be understood that steam is a chemical 
compound consisting of two parts of hydrogen and one part of 
oxygen. When the steam impinges upon the white hot brick- 
work of the furnace, the chemical union is broken, hydrogen and 
oxygen being liberated. The oxygen set free in this way is 
used in effecting the combustion of the oil, and the hydrogen is 
carried into the furnace. Now hydrogen is itself a combustible 
gas, and is burned in the furnace by the oxygen of the additional 
air which is supplied for this purpose. This combustion of 
hydrogen takes place further from the burner than the point 
at which the bulk of the oil is burned, and helps considerably in 
maintaining a uniform temperature. When the oil is atomized 
by a stream of compressed air, there is no hydrogen present to 
be burned in the furnace. 

Types of Oil-burning Furnaces. — Crude oil and kerosene are 
commonly used in oil-heated furnaces. To insure an unvary- 
ing temperature, the air and fuel pressures should be uniform. 
Two general types of oil-fired furnaces are shown in Figs. 3 
and 4. That shown in Fig. 3 is of what is called the over-fired 
type, in which the atomized gas from the oil burner passes into 
a space above the heating chamber, from which it is separated 
by an arch perforated by a number of openings through which 
the burning gas passes. This arrangement gives a uniform heat 
in the working chamber and the temperature is easily controlled. 
With a proper fuel supply and system of control, the temperature 
can be maintained within 25 degrees F. at any predetermined 
point between 600 and 1800 degrees F. Tests lasting for ij 
hour have been made in which the variation of temperature did 
not exceed 10 degrees. 



30 



HEAT-TREATMENT OF STEEL 



In Fig. 4 is shown the principles of a so-called under-fired 
furnace, in which the atomized gas from the oil first passes into 
chambers beneath the heating chamber. The combustion takes 
place in these lower chambers and the gas then passes through 
flues into the top of the heating chamber, from which it passes 
to the outlet flue, as indicated. It will be seen that the con- 
struction of a furnace of this type is simpler than is that of the 
over-fired type, so that the first cost is less. The cost of repairs 




Machinery 



Fig. 3. Diagrammatical Section showing the Principle of the Over-fired 
Type of Furnace 

is also smaller, but the fuel consumption is slightly greater and 
it is more difficult to maintain a uniform heat in all parts of the 
heating chamber than in the case of an over-fired furnace. When 
built in smaller sizes, however, the heat is easily controlled, and 
a furnace of this construction is suitable for tool hardening and 
tempering. 

Characteristics of Fuel Oils. — In an effort to determine the 
calorific values in British thermal units per pound of fuel oils, 
sixty-four samples of petroleum oils ranging from heavy crude 



HEATING FURNACES 



3 1 



oil to gasoline, representing the products of the principal oil 
fields in the United States, were examined for calorific power by 
combustion in oxygen in the Atwater-Mahler bomb calorimeter. 
It was found that the oils varied in fuel value from about 18,500 
to 21,100 B.T.U. per pound. In general, the decrease in calo- 
rific power with an increase in specific gravity is quite regular, 
so that the relation between the specific gravity and the heat 
value may be expressed approximately by means of a simple 
formula, as follows: 

B.T.U. = 18,650 + 40 (Number of Degrees Baume — 10). 




Diagrammatical Section showing the Principle of the 
Under-fired Type of Furnace 

When the heat values calculated from the specific gravity by 
means of this formula were compared with those actually de- 
termined by experiments, it was found that in one-ninth of the 
cases the difference was greater, and in eight-ninths it was less 
than one per cent. Hence, the heat value of commercially 
pure petroleum oils can be determined by the density of degrees 
Baume with sufficient accuracy for most practical purposes. 



32 HEAT-TREATMENT OF STEEL 

The heat value of oil is reduced by the presence of small per- 
centages of water. Therefore, if the oil contains water, it should 
be passed through a filtering tank before going to the burners. 
In this filtering tank the water settles to the bottom and can be 
easily drawn off. The oil should be heated before going to the 
filtering tank, as the water in the oil is more easily separated out 
of hot oil than cold oil, first, because heated oil offers less resist- 
ance to freeing the water, and, second, because there is a greater 
expansion of oil than water due to the heat, and the water, 
therefore, has a relatively greater specific gravity. 

Kerosene for Steel Heating Furnaces. — During the last few 
years the price of fuel oil has steadily advanced and in the fall, 
1 9 13, the oil refineries, in a number of states, notified industrial 
plants that they would be unable to supply fuel oil after a cer- 
tain date. At the present time it is almost impossible to enter 
into a contract with the refineries for a year's supply of this oil. 
This condition has caused many manufacturing plants to ana- 
lyze the situation and try to find a substitute for fuel oil without 
installing new equipment in the heat-treating department. The 
Continental Motor Mfg. Co., Detroit, Mich., has made numer- 
ous experiments with different kinds of oils and burners in order 
to be ready to meet the situation in case there will be a perma- 
nent shortage of fuel oil. Kerosene has proved most satisfac- 
tory, after many experiments, and the details given in the 
following show the results that have been obtained. 

The heat-treating room is of the most modern design and con- 
tains many unique features which are of interest. This depart- 
ment is housed in a fireproof building of structural steel and 
metal sash, and is entirely isolated from the other buildings. 
All auxiliary apparatus, such as oil and circulating pumps, air 
compressor and storage and cooling tanks, is located in the 
basement, so that the entire first floor is given up to the furnaces 
and quenching tanks. The furnaces were made by the Amer- 
ican Shop Equipment Co., and consist of four double-chamber 
case-hardening furnaces of semi-muffle type, with heating 
spaces 54 by 27 inches and 18 inches high. The combustion 
chambers are 27 inches wide by 7! inches high. The two burners 



HEATING FURNACES 33 

in front of each chamber are supplied with oil at 18 pounds pres- 
sure and air at if pound pressure. 

The furnaces show a very even temperature, and a labora- 
tory pyrometer fails to show over 10 degrees variation in any 
part of the heating chambers. Each furnace is equipped with 
both an indicating and a recording pyrometer, so that the oper- 
ator can tell at a glance the temperature of each chamber. To 
facilitate the handling of material, the hearths of all furnaces, 
packing tables, trucks, quenching tanks and other equipment 
are made the same height from the floor. The door openings 
are made the full width of the heating chamber and are counter- 
weighted. 

The quenching tanks are of special design and are located in 
the center and opposite the furnaces so that they are readily 
accessible. Water, brine and oil are used for quenching, and 
by means of cooling tanks and circulating pumps the quenching 
mediums are kept at a constant temperature. The pumps 
deliver the liquid in the bottom of the tanks by means of per- 
forated pipes which are protected by a wooden grating. This 
wooden grating also acts as a cushion for any parts which happen 
to fall into the tanks. The entire surface of the liquid is re- 
moved at a uniform rate by means of numerous outlets located 
near the top of the tank and connected with a common over- 
flow pipe. The rate of flow can be governed by means of a 
valve fitted to the inlet pipe near the floor. The oil quenching 
tanks have a cover which can be closed very quickly in case the 
oil ignites. The quenching mediums are circulated through 
coils and are cooled by means of cooling water which is varied 
to suit conditions. The circulating pumps are direct motor- 
driven centrifugal pumps, located in the basement. The liquid 
is fed by gravity to these pumps, which does away with the 
troublesome feature of priming. 

The oil is fed to the burners by direct-connected, motor- 
driven rotary pumps, in duplicate. The oil is pumped from a 
12,000-gallon tank located outside the building, and the surplus 
oil is returned to the storage tank by means of a relief valve and 
overflow. This system allows a constant pressure of oil at the 



34 



HEAT-TREATMENT OF STEEL 



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36 HEAT-TREATMENT OF STEEL 

burners at all times, and the fuel oil is entirely drained back to 
the storage tank when the heat-treating room is not in oper- 
ation. The elevation and plan, Figs. 5 and 6, show the storage 
tank and arrangement of the furnaces, oil and brine cooling 
coils, quenching tanks, etc. 

A General Electric turbo air compressor furnishes the neces- 
sary air for the burners at ij pound pressure. The air lines 
were carefully laid out in order to reduce the friction to a min- 
imum. The advantage of this type of air compressor is the 
constant pressure of air, regardless of the volume. 

Tests were repeated for ten successive days on both kerosene 
and fuel oil. Both oils burned uniformly and needed very little 
attention after the proper regulation. One of the chambers of 
a double furnace was used for these tests, and in order to accu- 
rately measure the oil, a tank of 60 gallons capacity was inde- 
pendently connected with one of the rotary pumps. The proper 
connections were made to maintain a constant pressure of oil, 
together with the necessary return piping for the overflow to 
the tank. A temperature record was obtained by means of 
smoke charts made on Bristol recording pyrometers. Both the 
indicating and recording pyrometers were carefully calibrated. 
The tests were made under actual working conditions and the 
same kind and quantity of material was heat-treated each day. 
The material placed in the chamber consisted of fifteen cam- 
shafts each weighing 21 pounds. Each cam-shaft was packed 
in a 3|-inch steel pipe with the carbonizing material. The 
total weight of each tube, with shaft and container, was 55 
pounds. All the tubes were placed on a frame made of narrow 
strips of bar steel. This frame held the steel pipes in position 
and made the total weight in each chamber 950 pounds. 

This department is only operated ten hours each day. The 
material is taken out at 5 P. M. and the furnaces are allowed to 
cool over night. In the morning the furnace pyrometers regis- 
ter about 700 degrees F. At 6.30 A. M. the burners are lighted 
and the material is placed in the furnaces which are continued 
in operation until about 5 P. M. The furnaces reach 1700 de- 
grees about 9 A. M., and they show a uniform heat throughout 



HEATING FURNACES 



37 



the working chamber about one hour later. The tests were 
made under these conditions. The furnaces would have shown 
higher efficiency if they had been run continuously twenty-four 
hours every day. 

The points noted in making these tests were: Time required 
for the furnace to reach 1700 degrees; time to obtain a uniform 
heat in the furnace; evenness of burning and regulation; and 
amount of oil consumed. The results of the tests follow: 



Time tests started 

Time required for chamber to reach 1700 
degrees F 

Time for chamber to reach uniform heat. 

Time test stopped 

Fuel consumption required for chamber 
to reach 1700 degrees F 

Additional gallons to reach uniform 
heats 

Number gallons for operating furnace 
6W hours after chamber reaches uni- 
form heat 

Number gallons per chamber, per hour 
after uniform heat is reached 

Number gallons to heat-treat one pound 
of metal after uniform heat is reached . 

Specific gravity of fuel 

B.T.U. per pound of fuel 

Temperature of fuel 



Fuel Oil 


Kerosene 


6.30 A.M. 


6.30 A.M. 


9.OO A.M. 

(2 hr. 30 min.) 

10. 15 A.M. 
(3 hr. 45 min.) 
4.45 P.M. 


8.45 A.M. 

(2 hr. 15 min.) 
9.45 A.M. 

(3 hr. 15 min.) 
4.15 P.M. 


26.5 gal. 


23 gal. 


3-5 gal. 


3 gal. 


18 gal. 


13 gal. 


2.77 gal. 


2 gal. 


0.057 gal. 
41 deg. Baume 
19,890 
65 deg. F. 


0.041 gal. 
50 deg. Baume 
19.917 
65 deg. F. 



The comparative cost of fuel based on these tests shows that 
kerosene is from 23 to 25 per cent higher than fuel oil, but the 
tests also show that there is a saving of thirty minutes' time 
for the furnace to reach a uniform heat, and a saving of nearly 
20 per cent in fuel, by using kerosene. Each burner has a |-inch 
oil line and a ij-inch air line and two burners are connected to 
each chamber. Power tests show that the blower requires 470 
watts for each burner, which is equivalent to 0.63 horsepower. 

Modern Types of Gas- and Oil-fired Furnaces. — In the last 
decade many improvements have been made in furnaces in which 
steel is heated, to obtain greater accuracy and uniformity in the 



38 HEAT-TREATMENT OF STEEL 

temperatures. Only a few years ago any temperature that was 
above the temperature at which steel would harden and below 
that at which it would become burnt, was considered good enough. 
In most steels this would cover a range of something like 300 de- 
grees F. Recent investigation, however, has shown that only a 
few degrees of variation in temperature between these two points 
makes considerable difference in the hardness, the elastic limit, 
the reduction of area, or the longevity of steel, as shown by 
fatigue tests. For instance, to heat steel 50 degrees above the 
transformation or critical point, or the point at which it should 
be quenched for hardening, shows a loss of something like 15 
per cent in these physical properties; and greater variations 
show correspondingly greater losses. Thus, while large fur- 
naces formerly had a variation of some 50 to 100 degrees in 
different parts of the heating chamber, the modern furnace 
must be so designed that the temperature will not vary more 
than 10 degrees between any two places, when the furnaces 
are operated at temperatures that are between 1400 and 1800 
degrees F. 

The interior construction of the furnaces has, therefore, been 
changed; appliances for pre-heating the fuel have been devised; 
gas and oil burners have been improved; automatic heat control 
instruments have been attached; and heat measuring instru- 
ments of various kinds have been brought into use to register 
and record the temperatures. Other improvements have also 
been made to reduce the fuel consumption; considerable study 
has been devoted to this part of furnace designs. 

The consumption of fuels has been studied by the Bureau of 
Mines, at Washington, D. C. One of the things conclusively 
proved is that a solid wall is better than a hollow wall, especially 
if the air space is near the outer side of the furnace. The gen- 
eral belief has been that air spaces built into the walls of a 
furnace would greatly reduce the amount of heat that was dissi- 
pated through the walls. The investigation mentioned proved, 
however, that while heat would travel slowly through air, be- 
cause it is a poor conductor, the heat would readily leap the air 
space by radiation, and thus a considerable percentage was lost 



HEATING FURNACES 



39 




40 



HEAT-TREATMENT OF STEEL 



that could have been saved if the air space had been filled with 
some solid non-conductor of heat. 

These results caused the Industrial Furnace Co., of Detroit, 
to design and build furnaces in the manner shown in Fig. 7. In 
these A is the cast-iron shell or outer wall of the furnace; B is 
mineral wool, or asbestos, used as a heating insulator; its thick- 
ness varies from 2 to 4 inches according to the size of the furnace; 
C is the inner fire-brick wall of the furnace; and D, the fire-brick 
floor. The burners are located at E, and the arrow heads show 




Machinery 



Fig. 8. Twin Furnace connected with a Conduit in order to make Use of 
the Heated Combustion Gases Twice 

the direction in which the flames and hot gases circulate. After 
passing through the heating chamber and giving up their heat, 
the spent gases pass through the vents V. At F is a sliding door 
that is typical of such furnaces, and at G the peep-hole in the 
door, while H is a shelf in front of the furnace. 

Another method of economizing fuel is shown in Fig. 8. Here 
the Industrial Furnace Co. has taken two furnaces, similar to 
the one shown in Fig. 7, and provided a conduit connecting the 
side of one with the bottom of the other. This conduit is lined 
with asbestos and fire brick, the same as the furnace, and can be 



HEATING FURNACES 41 

taken off at any time, so that the furnaces may be used as sep- 
arate units. In this twin furnace harrow springs are inserted in 
furnace 7, to the left, and heated to the correct hardening tem- 
perature which is here maintained. After the gases have done 
their work in furnace 7, they pass through conduit J to furnace 
K, to the right, and heat this to the correct temperature for 
drawing the temper of the springs. Thus the heat from the fuel 
is used the second time before it is allowed to escape to the 
atmosphere. In long furnaces several conduits are necessary to 
distribute the heat and make the temperature uniform in all 
parts of the heating chamber. The conduits should then be 
provided with dampers, so that the heat can easily be controlled. 
With only one conduit, however, the temperature within the 
heating chamber of the tempering furnace can be controlled by 
opening and closing the vent hole. 

The same principle has been used by the Garrett-Tilley Fur- 
nace Co., of New York, in a three-chambered furnace, each one 
of which is maintained at a different temperature. These are 
built in a single unit as shown in Fig. 9 ; that is, the three differ- 
ent heating chambers are built inside of one furnace shell. In 
one instance this type of furnace has been used for manufactur- 
ing leaf springs. In that case, the heating chamber L is used to 
heat the spring plates to the fabricating heat, which is about 
1800 degrees F. When taken from this fire the plates are bent 
to the correct shape to fit the leaf below, on which they have 
their bearing. After that they are inserted in the middle fur- 
nace or heating chamber M, to be heated to the hardening tem- 
perature, which is around 1500 degrees F. When heated to 
this temperature they are taken from furnace M and quenched 
in oil. After that they are inserted in the furnace or heating 
chamber N, and heated to the drawing temperature, which is 
about 750 degrees F. This allows an accurate control of the 
temperature in the three separate heating chambers, and each 
is maintained all day at its respective temperature of 1800, 
1500 and 750 degrees F. In this case fuel oil is used for heating 
the furnace, and pyrometers are used to measure the heat in each 
oven, so that the temperature can be kept at the correct point. 



42 



HEAT-TREATMENT OF STEEL 



The heating chambers in this furnace are about six feet in 
length, and it is especially designed to give a uniform tempera- 
ture in all parts. On the test run the variation between any 




SECTION Z-Z 




Machinery 



Fig. 9. Over-fired Furnace containing Three Heating Chambers arranged 
for Accurate Heat Control 

two places in each of the heating chambers was shown to be less 
than 10 degrees. This accuracy was obtained by over-firing 
and passing the heat through a honey-combed arch over the 



HEATING FURNACES 43 

heating chamber, as shown by the sectional view at in furnace 
L. This arch separates the combustion chamber W from the 
heating chamber L. The burners are located at S, and the flames 
enter the furnace at T where they strike a baffle plate R. This 
distributes them to both sides of the heating chamber. After 
filling the heating chamber, the hot gases pass through the 
openings in the honey-combed arch, as shown at U, and heat 
the oven in which the work is placed. The spent gases then 
leave the heating chamber through ports P, pass underneath 
the floor of the furnace and up through the vents V. Thus the 
top, sides and bottom of the heating chamber are kept at the 
same temperature throughout its entire length. This over-fired 
type of furnace has been used in furnaces with a single heating 
chamber as well as in those that have two and three heating 
chambers and has proved very successful. 

Still another improvement in oil-fired furnaces was recently 
patented by Walter S. Rockwell of The Rockwell Co., New York. 
This is shown in Fig. 10. It consists of a pipe coil through which 
the air is passed and pre-heated before it reaches the burners, 
where it is mixed with the fuel oil. This pre-heating coil A is 
located in front of the furnace, directly over door B, where it 
receives the heat which comes through the opening in which the 
work is inserted into the furnace. The plate C in front of the 
coil serves the purpose of protecting the furnace operator from 
the heat which comes through door opening B. One of these 
furnaces has been used by the Detroit-Timken Axle Co. for some 
time; it is claimed that the fuel consumption has been reduced 
by more than twenty-five per cent over that of furnaces that do 
not pre-heat the air. All or part of the waste gases can be made 
to pass through the door opening instead of through vents, and 
thus pre-heat the air to any degree that is desired. 

The air for combustion enters the coil, under pressure from 
a pump, through pipe D, which is provided with valve E to 
regulate this pressure, so that the air and fuel oil will have the 
proper mixture. The heated air leaves the coil through pipes F 
and enters burners H, where it is mixed with the fuel oil which 
flows to the burners through pipes /, its rate of flow being con- 



44 



HEAT-TREATMENT OF STEEL 




HEATING FURNACES 45 

trolled by valves K. The blast of hot gases then passes from 
burners H into heating chamber L to raise it to the correct 
temperature, and out through door B to heat the air in coil A . 

Advantages of Oil and Gas Furnaces. — Oil- and gas-fired 
furnaces are a great improvement over those that are fired with 
coal and coke, as with the latter it is impossible to keep the 
temperature in the furnace at a given point, and much of the 
heat is lost through the chimney which must be provided to 
carry away the smoke and gases. A large part of the furnace 
operator's time is taken up in shoveling in the coal or coke and 
carrying away the ashes, while the dust and dirt that accumu- 
lates from these operations is, to say the least, very disagreeable. 
Then again the steels heated in such furnaces are more liable to 
oxidize and scale, and to absorb some of the sulphur or other 
injurious elements that arise from the combustion. It is a well- 
known fact that steel absorbs such impurities readily when 
heated to the high temperatures required for hardening, forging 
and welding. For these reasons, furnaces using liquid fuels have 
improved the quality of the metal heated in them, effected a 
considerable saving in fuel consumption, and saved time by allow- 
ing the operator to give more of his attention to the heating of 
the metal. They have also effected a big improvement in the 
cleanliness of rooms in which furnaces are located. 

When gas and oil furnaces were first installed, a 50-degree vari- 
ation in the temperature during a day's run or in different parts 
of the furnace was considered quite good performance, but recent 
improvements have brought this to a point where a 10-degree 
variation is all that is allowed in high-class furnaces. This has 
made it possible to heat-treat steels at more accurate prede- 
termined temperatures, and thus give them greater strengths 
and resistance to fatigue. 

One of the greatest improvements that has been made for 
controlling the heat in the gas furnace is the temperature control 
instrument that is manufactured and attached to furnaces by 
the American Gas Furnace Co. This automatically increases 
and reduces the amount of gas and air that enters the burners 
and hence raises and lowers the flame that enters the furnace. 



4 6 



HEAT-TREATMENT OF STEEL 




HEATING FURNACES 47 

It is operated by a mechanism that is attached to the pyrometer. 
By means of a diaphragm this mechanism raises and lowers a 
sleeve containing gas and air ports, and thus increases or reduces 
the size of the port openings. With this instrument the heat 
inside the furnace can be kept within five degrees of the point 
at which the instrument is set, and the temperature can be main- 
tained within this narrow limit as long as the gas and air blast 
keeps flowing into the furnace. The oil-burning furnace must 
be regulated by an adjustment of the oil and air valves, by the 
furnace operator, as no instrument has yet been perfected that 
will automatically do this. Several individuals are working on 
this problem, however, and seem to have arrived at a solu- 
tion. Thus it will probably be but a short time before a similar 
instrument will be devised for automatically controlling the 
temperatures in furnaces using oil for fuel. 

Many improvements have also been made in oil- and gas- 
burning furnaces that heat liquid baths for raising the temper- 
ature of steel to the hardening temperatures. One of the im- 
provements made in an oil-burning furnace is shown in Fig. 11. 
This was designed by W. S. Quigley of the Quigley Furnace & 
Foundry Co. and uses a honey-combed arch similar to that 
shown in Fig. 9. The arch is used underneath a lead pot to sepa- 
rate the combustion chamber from the heating chamber and 
more evenly distribute the heat underneath the entire length of 
a five-foot lead pot. The flames from the burner enter opening 
A and strike the baffle plate B where they are broken up and 
distributed to both sides of combustion chamber C. The hot 
gases then pass through honey-combed openings D into heating 
chamber E and the spent gases leave the furnace through vent F. 

Salt hardening and tempering bath furnaces are finding more 
users every day, and oil tempering baths have been used for a 
long time and doubtless will be used to a great extent in the 
future. These can also be heated with this same design of 
furnace. Many of the fluid bath furnaces are constructed with- 
out the arch, and it is hardly applicable unless the length of the 
fluid pot is several times greater than the width. A great major- 
ity of lead and salt bath furnaces contain round pots and then 



48 



HEAT-TREATMENT OF STEEL 



the perforated arch is a detriment instead of an improvement. 
The design of such a furnace is shown in Fig. 12. 

Electrically-heated Furnaces. — Furnaces in which the heat 
is produced by electrical resistance are generally considered very 





Machinery 



Fig. 12. Simple Modern Type of Furnace for heating Lead or Salt Baths 

satisfactory for the heating of steel for hardening, but the cost 
of electricity exceeds that of liquid or gaseous fuels. A special 
chapter will be devoted to the various types of electrically- 



HEATING FURNACES 49 

heated furnaces that are employed. One type that is com- 
monly used derives its heat from a heavy, low-voltage current 
which passes through electrodes to resistance elements in the 
heating chamber. This type of furnace produces a uniform heat 
and the heat can also be accurately regulated. Electrically- 
heated furnaces are also used in conjunction with heating baths, 
the current being transmitted through a bath of metallic salts 
by two electrodes on opposite sides of the crucible. The con- 
ductivity of the salt is very small at normal temperatures, but 
at high temperatures, when the salt is in a molten condition, it 
offers but a slight resistance to the electric current, and, there- 
fore, when the bath is hot, it forms an electric conductor and 
each part of the bath produces its own heat. 

Solid Fuels for Steel-heating Furnaces. — Solid fuels, such as 
coal, coke, charcoal, etc., are used in many cases in hardening. 
A common type of solid fuel furnace is equipped with a grate 
upon which the fuel is burned and an arch above the grate 
which reflects the heat back to the plate that holds the steel 
to be heated. This plate should be so located that the flames 
do not come into direct contact with the steel, so as to oxidize 
and injure the finished surfaces. To prevent this, the steel is 
sometimes safeguarded by placing it inside of a clay or cast- 
iron retort which is encircled by the flames. For most purposes, 
the solid fuel type of furnace is less satisfactory than other types, 
because it is difficult to maintain a uniform temperature and 
the gases of combustion are liable to cause injury to the steel. 

Heating Steel in Liquid Baths. — The liquid baths commonly 
used for heating steel tools preparatory to hardening are molten 
lead, cyanide of potassium, barium chloride, a mixture of barium 
and potassium chloride and other metallic salts. The molten 
substance is retained in a crucible which is usually heated by gas 
or oil. The principal advantages of heating baths are as follows : 
No part of the work can be heated to a temperature above that 
of the bath; the temperature can be easily maintained at what- 
ever degree has proved, in practice, to give the best results; the 
submerged steel can be heated uniformly, and the finished sur- 
faces are protected against oxidation. 



50 HEAT-TREATMENT OF STEEL 

The Lead Bath. — The lead bath is extensively used, but is 
not adapted to the high temperatures required for hardening 
high-speed steel, as it begins to vaporize at about 1190 degrees F., 
and, if heated much above that point, rapidly volatilizes and 
gives off poisonous vapors; hence, lead furnaces should be 
equipped with hoods to carry away the fumes. Lead baths are 
especially adapted for heating small pieces which must be hard- 
ened in quantities. Gas is the most satisfactory fuel for heat- 
ing the crucible. It is important to use pure lead that is free 
from sulphur. The work should be pre-heated before plunging 
it into the molten lead. 

To prevent hot lead from sticking to parts heated in it, mix 
common whiting with wood alcohol, and paint the part that is 
to be heated. Water can be used instead of alcohol, but in that 
case the paint must be thoroughly dry, as otherwise the mois- 
ture will cause the lead to " fly." Another method is to make 
a thick paste according to the following formula: Pulverized 
charred leather, 1 pound; fine wheat flour, ij pound; fine table 
salt, 2 pounds. Coat the tool with this paste and heat slowly 
until dry, then proceed to harden. Still another method is to 
heat the work to a blue color, or about 600 degrees F., and then 
dip it in a strong solution of salt water, prior to heating in the 
lead bath. The lead is sometimes removed from parts having 
fine projections or teeth, by using a stiff brush just before immers- 
ing in the cooling bath. This is necessary to prevent the for- 
mation of soft spots. 

Melting pots for molten lead baths, etc., should, preferably, 
be made from seamless drawn steel rather than from cast iron. 
Experience has shown that the seamless pots will sometimes 
withstand six months' continuous service, whereas cast-iron pots 
will last, on an average, only a few days, under like conditions. 
Cast-steel melting pots, if properly made, are as durable as those 
made of seamless drawn steel. 

Cyanide of Potassium Bath. — Many steel hardeners prefer 
cyanide of potassium to lead, for heating steel cutting tools, 
dies, etc. When cyanide is used, the parts should be suspended 
from the side of the crucible by means of wires or wire cloth 



HEATING BATHS 5 1 

baskets, to prevent them from sinking to the bottom. Steel 
will not sink in a lead bath, as lead has a higher specific gravity 
than steel. Cyanide of potassium should be carefully used, as 
it is a violent poison. The fumes are very injurious, and the 
crucible should be enclosed with a hood connecting with a chim- 
ney or ventilating shaft. This bath is extensively used for 
hardening in gun shops, in order to harden parts and at the 
same time secure ornamental color effects. 

Barium-chloride Baths. — A temperature up to 2200 degrees 
F., and even higher, can be obtained by the barium-chloride bath, 
and this bath, therefore, is used to some extent for heating high- 
speed steel for hardening. There are certain disadvantages, 
however, met with in the use of barium chloride, and for this 
reason it has been discarded by many manufacturers. This 
subject is dealt with in greater detail in a following chapter, 
in connection with electrical hardening furnaces using barium- 
chloride baths. In Fig. 13 is shown a gas-heated furnace with 
a barium-chloride bath, which has been used by Wheelock, 
Lovejoy & Co. This furnace is used for heating high-speed 
steel, the operations in connection with its use being as follows: 

The tools to be hardened are first pre-heated, using a small 
gas furnace. The pre-heating saves time in the barium bath, 
and is absolutely necessary to avoid checking or cracking the 
tools, as will be conceded when it is remembered that the temper- 
ature of the barium bath is kept at between 2100 and 2200 de- 
grees F. After the tools are pre-heated, they are immersed in 
the barium bath, being suspended by an iron wire, or, in the case 
of small parts, in sheet nickel baskets. The reason for using 
sheet nickel for the baskets is that chloride of barium has a slight 
dissolving effect on iron, and the exposure of a large area of sheet 
iron in the bath would eventually destroy the baskets. Nickel 
is not affected to a perceptible extent, nor is the thin iron wire 
used to suspend ordinary tools. 

The temperature of the barium bath is regulated by a thermo- 
electric pyrometer. This instrument, shown at the left in Fig. 13, 
is similar to an ammeter or voltmeter, and the fire end is a thermo- 
electric couple. The heat of the bath affects the thermo-electric 



52 



HEAT-TREATMENT OF STEEL 



couple and generates a current that deflects the indicator of the 
indicating instrument to correspond with the temperature. For 
convenience in operation, the indicating instrument is provided 
with a double hand, one hand, A, being controlled by the tem- 
perature of the bath, while the other, B, is a marker set by the 
operator to indicate the temperature which he desires to carry. 
This marker is made with a disk at the end that covers a hole 
in the indicating hand when the two coincide, as they do when 
the temperature has reached the predetermined point. Thus, 



FIRE END OF 
PYROMETER 




&=P 



Machinery 



Fig. 13. Vertical Cross-section of Furnace used for Heating Barium- 
chloride Bath 

an operator whose eyes are dazzled by the bright light of the 
bath does not have to painfully study the graduations to see 
whether the pointer has reached the correct position, but by 
glancing at the instrument, he can readily determine when the 
indicator is directly beneath the marker referred to. 

The immersion of a piece pre-heated to a dull red immediately 
causes the indicator to drop, the temperature of the bath falling 
perhaps 30, 40 or even 50 degrees. The fall in temperature is 
due to absorption of heat by the piece, being the same as the 
refrigerating effect of a lump of ice thrown into a pot of boiling 



HEATING BATHS 53 

water, and several minutes may be required to raise the temper- 
ature of a large piece to the temperature that is required. For 
hardening " Blue Chip " steel, a temperature of from 2120 to 
2140 degrees F. has been found most suitable. After this tem- 
perature is attained, the part is allowed to soak for a few mo- 
ments, and then is lifted out and dipped into the cooling bath, 
which consists of cotton-seed oil agitated by compressed air 
admitted at the bottom. The cotton-seed oil is contained in a 
large iron barrel surrounded by water in a wooden tub. The 
part hardened is allowed to remain in the bath until it is quite 
cold. In practice, the operator hardens a batch and then re- 
moves the pieces by means of a wire basket hanging immersed 
in the oil. It is recommended that milling cutters, end mills, 
slitting saws, etc., made of " Blue Chip " steel, be used in gen- 
eral, without drawing the temper. However, an oil bath heated 
by gas and regulated by a thermometer should be provided for 
tempering such tools as require it. 

Chloride of barium is a white transparent salt (BaC^OEy 
which melts at a temperature of about 1700 degrees, the water 
of crystallization being driven off at a much lower heat. The 
salt volatilizes at an extremely high temperature, the loss at 
the temperature required for heating high-speed steel being 
negligible. The waste because of volatilization is, say, two 
pounds from a mass of barium weighing 75 pounds when held 
at a temperature between 2000 and 2300 degrees for five hours. 
This property of the chloride of barium bath of standing high 
temperatures without rapid volatilization is joined with others 
equally important. The piece heated is protected from the 
atmosphere during the heating period by the bath, of course, but 
the protective influence extends still further. A thin coating 
of barium clings to the piece when it is lifted out for immersion 
in the cooling bath, thus preventing oxidation. The coating of 
barium remaining when dipping prevents the coating of burned 
oil so troublesome to remove, so that on the whole the process 
probably produces the cleanest work of any bath known. The 
effect of the barium-chloride bath on the steel is, however, as 
already mentioned, not altogether satisfactory. 



54 HEAT-TREATMENT OF STEEL 

Fig. 13 shows what is considered an improved form of the 
furnace and crucible used for the chloride of barium bath. The 
common form of furnace and crucible in use employs a compara- 
tively shallow crucible, which necessitates making a joint be- 
tween the top of the crucible and the firebrick cover. This gives 
trouble by loosening and permitting the hot gases to escape 
around the edge of the crucible. The improved construction 
utilizes a deeper crucible, the top of which comes flush with the 
firebrick cover and simplifies the construction. The deep cru- 
cible also gives a greater volume of chloride of barium, conse- 
quently the refrigerating effect of the pre-heated steel parts, 
when immersed in the bath, is not so great. This illustration 
also shows the fire end, C, of the pyrometer immersed in the 
bath. It has been found advisable to employ crucibles made 
for steel melting, the ordinary graphite crucible used for brass 
melting giving trouble by flaking off into the barium. 

Barium-chloride Baths used for Carbon Steel. — When a 
barium-chloride heating bath is used for the lower temperatures 
required for carbon steels, it is mixed with chloride of potassium. 
For temperatures between 1400 and 1650 degrees F., use three 
parts of barium chloride and two parts of chloride of potassium. 
For higher temperatures, the amount of potassium chloride 
should be proportionately reduced. When temperatures of 
2000 degrees F. and over are required, pure barium chloride must 
be employed. In all cases, steel heated in barium chloride baths 
should be pre-heated to 600 or 800 degrees F. before being im- 
mersed in the bath. If temperatures below 1075 degrees F. are 
required, these may be obtained by using equal parts of potassium 
nitrate and sodium nitrate. This mixture sets at a temperature 
of 400 degrees F. and is used as a tempering bath, the range of 
heat obtained with this bath covering practically all the ordi- 
narily used tempering heats. 

Value of Pyrometer for Gaging the Heat. — All modern hard- 
ening rooms are now provided with pyrometers in order to 
insure uniformity of the heat in the hardening furnaces, and, 
consequently, uniform results in the hardened product. The lack 
of pyrometers, the failure to use pyrometers when provided, the 



HEATING BATHS 55 

hardening in charcoal furnaces insufficiently heated, and the 
heating of high-speed steel without pre-heating, are a few of 
the causes that produce ununiform results. It seems to be the 
general opinion among those who do not obtain uniform results 
when using pyrometers in hardening, that pyrometers are not 
of much use, that they do not give correct readings, and that 
as good or better results can be obtained by depending upon a 
man's experience in this particular work. It is true that many 
have met with difficulties in the use of pyrometers, but at the 
same time there is a remedy for this, and that is frequent cali- 
bration. While there is no question but that an experienced 
man's eye is a better judge of the heat in a furnace than an in- 
correctly calibrated pyrometer, it must be conceded that it is 
possible to keep the pyrometers in such condition that the read- 
ings are a great deal more accurate than any estimate of heat 
by the eye. 

One large firm making lathe and planer cutters for tool- 
holders has sent out thousands of cutters since the adoption of 
scientifically correct methods and the use of pyrometers with- 
out receiving practically any complaints, which indicates the 
possibilities of the modern methods. Furthermore, the experi- 
ence of this firm proves that with the proper heat-treatment any 
one of the high-grade high-speed steels is entirely satisfactory and 
that cutters made from any of these best brands cannot be told 
apart when in use; but the heat- treatment for each, of course, 
must be suited to that particular steel. In the hardening of 
high-speed steel, this firm pre-heats all the cutters in a low-heat 
furnace to a temperature of 1350 degrees F. This heating re- 
moves all the internal strains in the metal and puts it in the best 
possible condition for bringing it quickly to the high heat neces- 
sary for high-speed steel. Every cutter is treated in accordance 
with its sectional area and size, and when placed in the high-heat 
furnace it remains there for a length of time that has been deter- 
mined to be correct, for each size, by test. The pyrometers are 
used both for the low-heat and high-heat furnaces and these 
pyrometers are tested twice a week. It has been found that 
this is as long as a pyrometer can be safely employed without 



56 HEAT-TREATMENT OF STEEL 

checking. The hardening room of this firm is provided with a 
specially made clock which starts on the pull of a lever by the 
man running the furnace, the clock having previously been set 
to indicate the proper length of time required for the cutter being 
treated. At the end of the predetermined time, the clock rings 
a bell and stops, and the operator takes out a cutter and puts 
it into the proper cooling medium. 

The experience of this concern indicates that it is necessary 
to have the high-heat furnace at a temperature above the melt- 
ing point of the high-speed steel in order to get the best results. 
This, of course, requires absolute accuracy in the length of time 
that the work is permitted to remain in the furnace, as a minute 
too much would ruin the work, and too short a time would not 
give sufficient heat. Another reason for this high heat in the 
furnace is that to get the best results from high-speed steel, it 
is necessary, according to the experience of this firm, to raise 
the heat from 1350 degrees F. to the highest heat required in a 
very short period of time. The essentials for proper heat- treat- 
ment of high-speed steel are, therefore: A high-grade quality of 
high-speed steel, pre-heating and rapid rise in the temperature 
of the steel from the pre-heating temperature to the proper 
quenching heat in a high-heat furnace, the use of an accurately 
calibrated pyrometer, and a correct time chart indicating the 
length of time that each particular piece should remain in the 
high-heat furnace. 

Thermo-electric Pyrometer. — The most commonly used py- 
rometers for the heat-treatment of steel are of the thermo-elec- 
tric type. In this type, temperature variations are determined 
by the measurement of an electric current generated by the 
action of heat on the junction of two dissimilar metals; that 
is, when one junction of the thermo-couple has a temperature 
different from the other, a current is developed and a meter 
indicates the temperature, the relation between the strength 
of current and the temperature being constant. The thermo- 
couple and the meter form the essential parts. The two dissimi- 
lar metals composing the thermo-couple are connected at one 
end, which is called the " hot end," and placed in the furnace or 



PYROMETERS 57 

heated place, the temperature of which is required. Except at 
the hot end, the two wires or elements do not touch. The free 
ends, called the " cold end," are kept away from the heat. 
When the hot end is heated, the intensity of the current gener- 
ated depends upon the difference between the temperature of 
the hot and cold ends. The meter is connected to the cold end 
and shows the value of the current in degrees Fahrenheit or 
Centigrade. Some pyrometers of this type may be used, inter- 
mittently, for temperatures up to 3000 degrees F. 

Resistance Pyrometer. — The variation in electric conductiv- 
ity due to changes in temperature is the principle upon which 
the resistance pyrometer is based. This type is very accurate 
for temperatures below 1600 degrees F., but should not be used 
continuously for higher temperatures. The maximum temper- 
ature is about 2200 degrees F. The thermo-electric type is pref- 
erable for indicating high-speed steel hardening temperatures, 
etc., because the resistance type will not stand exposure to in- 
tense heats, except for short periods. 

Different Instruments for Measuring Temperatures. — A 
comparative table of the various types of thermometers and 
pyrometers in use, explaining the general principles upon which 
each depends for its working, and stating the limits of tem- 
perature between which each type may be used is given here- 
with. This table is particularly valuable on account of the 
concise form in which the information it contains has been pre- 
sented. At the present time pyrometry is becoming an important 
subject in industrial life, and is not any longer confined to the 
scientific laboratory only. 

Calibration of Pyrometers. — Pyrometers should occasionally 
be compared with a standard pyrometer, or be calibrated in some 
other way. (The Government Bureau of Standards at Wash- 
ington will test and calibrate temperature gages for a moderate 
fee.) The following methods of testing the accuracy of thermo- 
electric pyrometers are recommended by the Hoskins Mfg. 
Company: The most satisfactory method is to compare the 
installed pyrometer with an accurate " check pyrometer " under 
service conditions. This can be done by placing the thermo- 



58 



HEAT-TREATMENT OF STEEL 



Types of Heat Measuring Instruments in General Use 






~~ 


Range in 
Degrees 


Class 


General Characteristics 
of Action 


Type 


Fahrenheit 

over which 

they can be 

used 


Expansion 


Change in volume 


Gas 


32 to 1800 




or length of a 


Mercury, Jena 






body with tem- 


glass and nitro- 






perature. 


gen. . . 


—40 to 900 
—35o to 




Glass and spirit 






or petrol 


+ IOO 






Unequal expan- 








sion of metal 








rods 


32 to 900 






Contraction of 






porcelain 


32 to 3250 


Transpiration 


Flow of gases 


TheUehling 


32 to 2900 


and Viscosity- 


through capil- 
lary tubes or 
small apertures. 






Thermo-electric 


Electromotive 
force developed 


Galvanometric 


32 to 2900 




by the difference 


Potentiometric . . . 


32 to 2900 




in temperature 








of two thermo- 








electric junc- 








tions. 






Electric Resist- 


Increase in electric 


Direct reading on 




ance 


resistance of a 


indicator or 






wire with tem- 


bridge and gal- 






perature. 


vanometer 


32 to 2200 


Radiation 


Heat radiated by 


Thermo-couple in 






hot bodies. 


focus of mirror . . 


32 to 18,000 






Bolometer 


32 to 18,000 


Optical 


Change in bright- 


Photometric com- 






ness or in wave- 


parison 


32 to 3600 




length of the 


Incandescent fil- 




light emitted. 


ament in tele- 








scope 


32 to 3600 






Nicol with quartz 






plate and ana- 








lyzer 


32 to 3600 




Calorimetric 


Specific heat of a 


Copper or plati- 






body raised to 


num ball with 






a high tempera- 


water vessel .... 


32 to 2700 




ture. 






Fusion 


Unequal fusibility 


Alloys of various 






of various metals 


fusibilities 


32 to 3600 




or earthenware 








blocks. 







PYROMETERS 59 

couple of the check pyrometer in the same protecting tube with 
the service thermo-couple, being careful to see that both couples 
are inserted to the full depth of the tube; then by comparing 
the readings of the two meters, the accuracy of the service pyrom- 
eter can be determined. The check pyrometer should also be 
calibrated occasionally. For a low-reading type, the calibration 
may be done by placing the thermo-couple in a heated oil bath 
and comparing the readings of the meter with those of a high- 
grade mercury thermometer having a range up to 150 degrees C. 
(302 degrees F.) . The vessel containing the oil should be, approx- 
imately, 6 inches deep, and from 4 to 6 inches in diameter, and 
be heated by a gas burner or an electric hot-plate. The oil 
should be stirred constantly to insure a uniform temperature 
throughout. Any oil having a high flash point, or some of the 
waxes, such as paraffin, beeswax, etc., may be used. 

Calibrating by the Melting Point of Copper. — For calibrat- 
ing pyrometers for temperatures above a red heat, the welded or 
" hot end " of the thermo-couple should be covered with a tight 
winding of No. 14 or 16 B. & S. gage, standard melting-point 
wire. The couple should then be inserted in a tube furnace with 
the welded end approximately in the center. The furnace 
should be of the required heat before inserting the couple, and 
should be kept at a temperature approximately 100 degrees F. 
higher than the melting point of the calibrating wire. The 
pointer of the meter will then move up the scale with a gradually 
decreasing speed until the calibrating wire begins to melt, when 
the pointer will come to rest. After the wire has melted, the 
pointer will again move upward. Pure copper wire, under oxidiz- 
ing conditions, melts at 1065 degrees C. (1949 degrees F.), and 
pure zinc wire, at 419 degrees C. (786 degrees F.). In order to 
have a strictly oxidizing atmosphere, an open-end electric fur- 
nace should be used for calibrating. With this method of cali- 
brating, care should be taken not to have the furnace temperature 
too far above the melting point of the calibrating wire, because 
the pointer will move so rapidly and the melting will be of such 
short duration that the temporary pause of the pointer may not 
be observed. 



<6o HEAT-TREATMENT OF STEEL 

Calibrating by the Freezing Point of Melted Salts. — A very 
satisfactory way of calibrating pyrometers is by using the " freez- 
ing points " of melted salts. Pure common salt (NaCl) is melted 
in a pure graphite crucible. When the salt has been raised to a 
temperature of ioo to 200 degrees F. above its melting point, 
the bare welded end of the thermo-couple is inserted to a depth 
of 2 or 3 inches. The crucible is then removed from the furnace 
and allowed to cool. The pointer on the meter will drop gradu- 
ally until the salt begins to freeze or solidify; then the pointer 
will stop until the salt is frozen. The freezing point of pure salt 
is taken at 800 degrees C. (1472 deg. F.). After calibrating and 
before being further used, the couple end should be washed in 
hot water to remove all traces of the salt, as otherwise the couple 
will deteriorate rapidly, especially when heated considerably 
above the melting point of salt in an open furnace. When cali- 
brating pyrometers, care should be taken that the zero setting 
of the meter agrees with the cold end of the couple, which is 
always kept away from the heat and generally at the temperature 
of the outside air. The following table gives the latest available 
data by the Bureau of Standards on certain substances which 
may be used for calibrating pyrometers. 

Water boils at 100 deg. C. (212 deg. F.) 

Tin freezes at 232 deg. C. (450 deg. F.) 

Zinc freezes at 419 deg. C. (786 deg. F.) 

Common salt freezes at. . . . 800 deg. C. (1472 deg. F.) 
Copper, in oxidizing atmos- 
phere, freezes at 1065 deg. C. (1949 deg. F.) 

Correcting the Thermo-couple. — If the pyrometer gives too 
high a reading, the correction is made by increasing the adjust- 
ing resistance, and if the reading is too low, the resistance is de- 
creased. The adjusting resistance of a Hoskins pyrometer is 
No. 26 B. & S. gage wire, which has a low temperature coefficient 
and is wound on a composition spool located within the handle 
of the thermo-couple. In changing this resistance, the connec- 
tion should always be carefully soldered and the different turns 
insulated from one another. This changing of the resistance 
should not be attempted by an inexperienced person. 



CHAPTER III 
QUENCHING AND TEMPERING 

While there has been a great deal published regarding the 
hardening and tempering of steel and the furnaces used, there is 
but little detailed information available regarding the baths used 
for these operations. The time has long since passed when each 
hardener had his own carefully guarded secrets regarding the 
composition of quenching baths. On the other hand, the time 
has also passed when it was a common belief that to harden a 
piece of steel it was only necessary to cool it off more or less 
rapidly in almost any kind of cooling medium; it has been found 
that the cooling mediums for hardening and the heating mediums 
or baths for tempering do, after all, play quite an important 
part both as regards economy and the efficiency of the tools 
treated. 

It is the intention in this chapter to outline the methods which 
have proved most successful, and to give the compositions of 
baths which from long experience in connection with hardening 
operations have proved to give the best all around results and 
to be the most economical; in many cases they have not been 
the cheapest in initial cost, but nevertheless are most economical, 
because of the better results obtained with the tools treated and 
the greater length of time that the baths could be used before 
deteriorating. A description of such receptacles — cooling 
tanks and tempering furnaces — for the treatment of steel as 
have proved to be the best for all around purposes has also been 
included. 

Characteristics of Quenching Baths. — No matter what the 
composition of a quenching bath, to insure uniform hardening 
the temperature of the bath must be kept constant, so that suc- 
cessive pieces of steel or tools quenched will be acted upon by 
baths of the same heat. The necessity of a uniform temper- 

61 



62 HEAT-TREATMENT OF STEEL 

ature for a quenching bath will be readily understood by refer- 
ence to ordinary water for a cooling bath; everyone having any 
knowledge of the subject knows that a tool quenched in such a 
bath at room temperature will come out much harder than if 
quenched when the water is at the boiling point. In fact, it is 
well known that one way of partially annealing steel is by plung- 
ing it at a red heat into hot water. The same difference in hard- 
ness will result when using any quenching bath at different 
temperatures, and hence no dependable results can be obtained 
unless means are taken for keeping these baths at a uniform 
heat. 

When using quenching baths of different composition the tools 
quenched will vary in hardness. This is due mainly to the differ- 
ence in heat-dissipating power of the different baths. Thus a 
tool hardened at the same temperature in water and brine will 
come out harder when quenched in brine; the greater the con- 
ductivity of the bath the quicker the cooling. The general 
opinion, today, is that the composition of a quenching bath is 
of small importance as long as the bath cools the pieces rapidly. 
Those who have made a study of the subject have found differ- 
ent opinions regarding the same quenching bath by different 
users, and a good many quenching fluids have been condemned 
owing to improper heating which in turn was due, in many 
cases, to improperly built furnaces. As an example may be 
cited an oven furnace with which the user once had trouble. 
Owing to faulty construction of this furnace, more air was let 
into the heating chamber of the furnace than could be taken care 
of by the fuel oil; after having condemned first the steel and 
then the quenching bath, and then trying one quenching bath 
after another with the same results, it was suggested that the 
" heating " did not look just right, and an expert was called in 
to find out what the trouble was. After much experimenting 
with the burners and the furnace itself good results were finally 
obtained. The difficulty seemed to be that the oxygen of the 
air attacked the steel and formed oxide of iron on the surface of 
the tools, which consequently had a soft scale on the outside. 

Those who are skeptical as to there being any difference in 



QUENCHING AND TEMPERING 63 

the effect on steel of cooling baths of different composition will 
readily admit that it is advantageous to use baths free from 
oxygen and from ingredients that tend to oxidize. Quenching 
baths should be uniform; good tool steels of high carbon are very 
sensitive to differences in both water and oils. Water for hard- 
ening tool steel should be soft; entirely different and very un- 
satisfactory results will be obtained when using hard water. 
While different quenching oils show less difference in the results 
obtained, vegetable and animal oils will give somewhat different 
degrees of hardness depending upon the sources from which they 
are obtained. One cannot be too careful in the selection of 
water, as it is likely to contain many impurities. If it contains 
greasy matters, it may not harden steel at all, whereas if it con- 
tains certain acids, it will be likely to make the tools quenched 
in it brittle and even crack them. 

Quenching Baths for Specific Purposes. — Clear cold water 
is commonly employed for ordinary carbon steel, and brine is 
sometimes substituted to increase the degree of hardness. Sperm 
and lard oil baths are used for hardening springs, and raw lin- 
seed oil is excellent for cutters and other small tools. The effect 
of a bath upon steel depends upon its composition, temperature 
and volume. The bath should be amply large to dissipate the 
heat rapidly, and the temperature should be kept about con- 
stant, so that successive pieces will be cooled at the same rate. 
Greater hardness is obtained from quenching in salt brine and 
less in oil, than is obtained by the use of water. This is due to 
the difference in the heat-dissipating qualities of these substances. 
If thin pieces are plunged into brine, there is danger of crack- 
ing, owing to the suddenness of the cooling. 

A quenching solution of 3 per cent sulphuric acid and 97 per 
cent of water will make hardened carbon steel tools come out of 
the quenching bath bright and clean. This bath is sometimes 
used for drills and reamers which are not to be polished in the 
flutes after hardening. Another method of cleaning drills and 
similar tools after hardening is to pickle them in a solution of 
1 part hydrochloric acid and 9 parts of water. Still another 
method is to use a heating bath consisting of 2 parts barium 



64 HEAT-TREATMENT OF STEEL 

chloride and 3 parts potassium chloride. This method is satis- 
factory for reamers and tools which are not to be polished in the 
flutes after hardening. A small quantity of sal-ammoniac added 
to an oil-bath also has a tendency to make the tools come out 
clean from the bath. 

That the temperature of the hardening bath has a great deal to 
do with the hardness obtained has been proved by actual tests. 
In certain experiments a bar quenched at 41 degrees F. showed 
a scleroscopic hardness of 101. A piece from the same bar 
quenched at 75 degrees F. had a hardness of 96, while, when the 
temperature of the water was raised to 124 degrees F., the bar 
was decidedly soft, having a hardness of only 83. The higher 
the temperature of the quenching water, the more nearly does its 
effect approach that of oil, and if boiling water is used for quench- 
ing, it will have an effect even more gentle than that of oil; in 
fact, it will leave the steel nearly soft. With oil baths, the 
temperature changes have little effect upon the degree of hard- 
ness. Parts of irregular shape are sometimes quenched in a 
water bath that has been warmed somewhat to prevent sudden 
cooling and cracking. A water bath having one or two inches 
of oil on top is sometimes employed to advantage for tools made 
of high-carbon steel, as the oil through which the work first 
passes reduces the sudden action of the water. 

Irregularly shaped parts should be immersed so that the heav- 
iest or thickest section enters the bath first. After immersion, 
the part to be hardened should be agitated in the bath; the 
agitation reduces the tendency of the formation of a vapor coat- 
ing on certain surfaces, and a more uniform rate of cooling is 
obtained. The work should never be dropped to the bottom of 
the bath until quite cool. High-speed steel is cooled for harden- 
ing either by means of an air blast or an oil bath. Both fresh 
and salt water are also used, although, as a general rule, water 
should not be used for high-speed steel. Various oils, such as 
cotton-seed, linseed, lard, whale oil, kerosene, etc., are also em- 
ployed; many prefer cotton-seed oil. Linseed has the objec- 
tion of becoming gummy, and lard oil has a tendency to become 
rancid. Whale oil or fish oil gives satisfactory results, but has 



QUENCHING AND TEMPERING 65 

offensive odors, although this can be overcome by the addition 
of about three per cent of heavy " tempering " oil. 

List of Quenching Baths. — (1) Water — soft — preferably 
distilled; good tool steel should require no mixture added to 
pure water. (2) Salt added to water; will produce a harder 
" scale " than if quenched in plain water. (3) Sea (salt) water 
— the keenest natural water for hardening. (4) Water as under 
(1), containing soap. (5) Sweet milk. (6) Mercury. (7) Car- 
bonate of lime. (8) Wax. (9) Tallow. (10) Air — mostly 
used for high-speed steel; mere exposure, however, is in many 
cases and on many steels not sufficient to produce hardness and 
an air blast is necessary, as this furnishes cool air in rapid motion. 
(11) Oils such as cotton-seed, linseed, whale, fish, lard, lard and 
paraffine mixed, special quenching oils, etc. Milk, mercury, 
carbonate of lime, wax and tallow are generally used for special 
purposes only. 

The following list of oils and names of firms supplying them 
is given for the sake of convenience. The firms mentioned 
are reliable and their oils have been thoroughly tried out in 
comparison with other makes and have proved to be superior; 
opinions may, of course, differ in this respect and no doubt 
there are many oils that have not been tried that may be as 
good. 

Cotton-seed oil — Union Oil Co., Providence, R. I. ; Underhay 
Oil Co., Boston, Mass. 

Linseed oil — Spencer Kellogg & Sons, Inc., Buffalo, N. Y. 

Whale oil — no difference found between two different kinds. 

Fish oil — only one kind tried. 

Lard oil — W. B. Bleecker, Albany, N. Y.; E. F. Houghton 
& Co., Pittsburg, Pa. 

Paraffine oil — Underhay Oil Co., Boston, Mass. 

Special quenching oil — E. F. Houghton & Co., Pittsburg, 
Pa. Very good and cheap. While this may possibly deteriorate 
somewhat faster than some of the others mentioned it will prove 
very economical. 

The order of the intensity with which various cooling baths 
will harden steel of about 0.90 to 1.00 per cent carbon is as fol- 



66 HEAT-TREATMENT OF STEEL 

lows: Mercury, carbonate of lime, pure water, water containing 
soap, sweet milk, different kinds of oils, tallow and wax. In all 
cases, except possibly the oils, tallow and wax, it must be remem- 
bered that the tools become harder as the temperature of the 
bath becomes lower. 

Receptacles and Tanks used in Quenching. — The main 
point to be considered in a quenching bath is, as mentioned, 
to keep it at a uniform temperature so that successive pieces 
quenched will be subjected to the same heat. The next consider- 
ation is to keep the bath agitated, so that it will not be of differ- 
ent temperatures in different places; if thoroughly agitated and 
kept in motion, as is the case with the bath shown in Fig. i, 
it is not even necessary to keep the pieces in motion in the bath, 
as steam will not be likely to form around the pieces quenched. 
Experience has proved that if a piece is held still in a thoroughly 
agitated bath, it will come out much straighter than if it has been 
moved around in an unagitated bath. This is an important 
consideration, especially when hardening long pieces. It is ; be- 
sides, no easy matter to keep heavy and long pieces in motion 
unless it be done by mechanical means. 

In Fig. i is shown a water or brine tank for quenching baths. 
Water is forced by a pump or other means through the supply 
pipe into the intermediate space between the outer and inner 
tank. From the intermediate space it is forced into the inner 
tank through holes as indicated. The water returns to the 
storage tank by overflowing from the inner tank into the outer 
one and then through the overflow pipe as indicated. In Fig. 2 
is shown another water or brine tank of a more common type. 
In this case the water or brine is pumped from the storage tank 
and continuously returned to it. If the storage tank contains 
a large volume of water, there is no need of a special means for 
cooling. Otherwise, arrangements must be made for cooling 
the water after it has passed through the tank. The bath is 
agitated by the force with which the water is pumped into it. 
The holes at A are drilled on an angle, so as to throw the water 
toward the center of the tank. In Fig. 3 is shown an oil quench- 
ing tank in which water is circulated in an outer surrounding 



QUENCHING AND TEMPERING 



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68 



HEAT-TREATMENT OF STEEL 



tank for keeping the oil bath cool. Air is forced into the oil 
bath to keep it agitated. 

Fig. 4 shows a water and oil tank combined. The oil is kept 
cool by a coil passing through it in which water is circulated, 
which later passes into the water tank. The water and oil bath 
in this case is not agitated. 

Fig. 5 shows the ordinary type of quenching tank cooled by 
water forced through a coil of pipe. This can be used for either 
oil, water or brine. Fig. 6 shows a similar type of quenching 
tank, but with two coils of pipe. Water flows through one of 



OVERFLOW— RETURN 
TO STORAGE TANK 




Fig. 4. Water and Oil Tank Combined 

these and steam through the other. By this means it is possible 
to keep the bath at a constant temperature. 

Tempering. — The object of tempering is to reduce the brittle- 
ness in hardened steel, and to remove the internal strains caused 
by the sudden cooling in the quenching bath. The tempering 
process consists in heating the piece of work, by various means, 
to a certain temperature, and permitting it to cool off gradually. 
The degree of heat to which the tool to be tempered is heated 
determines the degree of toughness — and unfortunately also 
the degree of softness — it has attained; the higher the temper- 
ing heat, the less brittle, and also the less hard, will the tool be. 

Tempering by the Color Method. — When steel is tempered 
by the color method, the tempering heat is gaged by the colors 



QUENCHING AND TEMPERING 



69 



formed on the surface as the heat increases. First the surface 
is brightened to reveal the color changes, and then the steel is 
heated either by placing it upon a piece of red-hot metal, a gas- 
heated plate or in any other available way. As the temper 
increases, various colors appear on the brightened surface. First 
there is a faint yellow which blends into straw, then light brown, 
dark brown, purple, blue and dark blue, with various inter- 




Fig. 5. Ordinary Type of 
Quenching Tank 



Fig. 6. Oil-quenching Tank with 
Water and Steam Coils 



mediate shades. The temperatures corresponding to the differ- 
ent colors and shades are given in the table on temperatures and 
colors for tempering. Turning and planing tools, chisels, etc., 
are commonly tempered by first heating the cutting end to a 
cherry-red, and then quenching the part to be hardened. When 
the tool is removed from the bath, the heat remaining in the un- 
quenched part raises the temperature of the cooled cutting end 



7° 



HEAT-TREATMENT OF STEEL 



until the desired color (which will show on a brightened surface) 
is obtained, after which the entire tool is quenched. The fore- 
going methods are convenient, especially when only a few tools 
are to be treated, but the color method of gaging temperatures is 
not dependable, as the color is affected, to some extent, by the 
composition of the metal. The modern method of tempering, 
especially in quantity, is to heat the hardened parts to the re- 
quired temperature in a bath of molten lead, heated oil or other 
liquids; the parts are then removed from the bath and quenched. 
The bath method makes it possible to heat the work uniformly, 
and to a given temperature within close limits. 
Colors for Tempering 



Color 


Degrees F. 


Degrees C 


Very pale yellow 


430 
440 

4SO 
460 
470 
480 
490 
500 
5IO 
520 
S30 
S40 
S50 
56o 
570 


221 
227 
232 
238 

243 
249 

254 
260 
266 
271 
277 
282 
288 
293 
299 


Light yellow 


Pale straw-yellow 


Straw-yellow 


Deep straw-yellow 


Dark yellow 


Yellow-brown 


Brown-yellow 


Spotted red-brown 


Brown-purple 


Light purple 


Full purple 


Dark purple 


Full blue 


Dark blue 





Baths used for Tempering. — As the object of tempering is 
simply to reduce the brittleness and to remove internal strains 
caused by sudden cooling in quenching, the composition of a 
tempering bath is of little importance compared with that of 
a quenching bath when considering the effect upon the pieces 
treated. Aside from the operator's convenience and possible 
bad effects upon his health, the different baths used for this 
operation must be considered with regard to initial cost, lasting 
quality, effects on finish, etc. 

While oil is the most widely used medium for tempering tools 
in quantities, other means and methods are employed, especially 
by those who have tools in small quantities to temper, when the 



QUENCHING AND TEMPERING 7 1 

expense of installing and running an oil tempering furnace would 
not be warranted. Of these methods we first find the one al- 
ready described, used by the old-style tool hardener, of only 
partly cooling the tool when quenching it, then quickly with- 
drawing it, polishing off the working surface, and then letting 
the heat which remains in the tool produce the required temper 
as judged by the color. 

The sand bath is another frequently used medium for temper- 
ing, the sand being deposited on an iron plate and heated; by 
the use of this method a piece to be tempered can be given differ- 
ent tempers throughout its length, as, for example, rivet hole 
punches; these are placed endwise — bottom down — in the 
sand about two-thirds projecting outside the sand into the air 



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Fig. 7. Arrangement used for Sand Tempering 

(see Fig. 7). It is readily seen that the nearer the bottom of 
the sand bath, the higher the heat, and the punch so placed, 
when tempered right, will have the bottom soft — a deep dark 
blue — the neck a very dark straw, and the working part of the 
punch on top a light straw color; thus there is a gradual increase 
in hardness from the bottom up. Pieces so drawn must previ- 
ously have been polished, and the temper is judged by the color. 
When the pieces have attained the right color they are, of course, 
cooled off, generally in water or oil. A plate without sand simi- 
larly heated can also be used, but it is not as satisfactory. 

A plate arranged as shown in Fig. 8 will be found very con- 
venient when drawing small, round pieces The pieces are 
rolled on the inclined plate which is heated as indicated. The 
length of time the work is in contact with the plate can be regu- 
lated by adjusting the amount of the incline, as well as the loca- 



72 



HEAT-TREATMENT OF STEEL 



tion of the " stop." This arrangement can also be used for 
such work as punches, etc., in which case the plate, of course, 
should stand level and not in an inclined position. 

Another frequently used tempering medium is hot air, the 
temper in this case also being judged by the color. For this 
method of tempering special furnaces should be employed in 
order to get uniform results. This method is used more espe- 
cially for small and light work in quantities and where the color 
has to be bright and clear. While all of these methods have the 
advantage of enabling one to actually see the temper given to 
tools treated, the oil tempering bath is the one mostly used owing 
to its economy. 



WORK TO BE TEMPERED 




W^WP^W 



COOLING 
BATH 



Machinenj 



Fig. 8. Tempering Arrangement utilizing an Inclined Plate on which 
the Objects roll down 

The two main points to be considered when using an oil temper- 
ing furnace are : first, to have the heat uniform throughout (not 
hotter where the burners or flames are in contact with the walls 
of the furnace) ; and second, to leave the pieces to be tempered 
in the oil long enough to have attained the heat of the oil through- 
out when taken out. The first point can be taken care of, as far 
as possible, by proper construction of the furnace; the second 
can best be taken care of by immersing the pieces to be tempered 
in the oil before starting to heat, and letting the pieces remain 
in the oil and be heated with it to the temperature required. 
In such a case, one should, of course, have more than one furnace, 
or else after each operation take the hot oil out and refill the tank 
with cold. The method described is very much better than 
the one frequently used of immersing the pieces in a bath which 



QUENCHING AND TEMPERING 73 

already has the required temperature and then letting them re- 
main long enough to attain the heat of the bath throughout, as a 
furnace yet has to be designed which will maintain a uniform heat 
for even as short a time as is required for this operation. Fur- 
thermore, it is not necessary that a piece to be tempered be held 
in the bath a certain length of time at the required temperature ; 
the temperature desired need only be maintained long enough 
to insure that the piece has been evenly heated throughout. 

There are tempering oils on the market claimed to have a 
flash test of 750 degrees, but it is doubtful if they ever have been 
found to stand this test. Heavy black cylinder oil has been 
found to stand a flash test of 725 degrees. Therefore, when 
tempering to high heats, or, rather, when tempering to higher 
heats than the flash point of any tempering oils (650 to 700 de- 
grees F.) some other tempering fluid than oil must be used. 
Lead is the one usually employed. As it is impracticable when 
using lead to let the pieces to be tempered be heated up with the 
lead, they must be immersed at the predetermined temperature 
and kept there until heated evenly throughout to the same 
temperature as the lead. It is claimed by many that it is easier 
to maintain a uniform heat in a lead bath than in an oil bath, 
but it has been found that, owing to the lead not circulating 
as readily, the temperature may vary considerably in different 
parts of the bath, and hence it is not very reliable. 

Salt is another medium frequently employed for tempering 
heats between 800 and 875 degrees F. Salt fuses at 800 de- 
grees F., but when immersing the pieces to be tempered the salt 
will immediately solidify around the cold pieces. When these 
are heated to 800 degrees, the salt will melt and the pieces should 
be withdrawn. This is not reliable, however, as the pieces, 
especially if large, will not have had time to be heated through 
before the salt melts. If a higher temper is required, it is, of 
course, only necessary to let the pieces remain in the bath and 
get the readings of the heat from a pyrometer. In all these 
methods, it is questionable if it is good practice to suddenly im- 
merse cold pieces to be drawn into baths of such high temper- 
atures. When a lower temper is required, and an oil tempering 



74 



HEAT-TREATMENT OF STEEL 



bath or furnace is not available, alloys of lead and tin can be 
used for as low heats as 400 degrees and of lead and antimony 
for 500 degrees. However, this involves the inconvenience of 
keeping a large number of different alloys on hand, if it is desired 
to vary the temper heats. The following table for different 
alloys which melt at the temperatures given was compiled by 
Mr. 0. M. Becker. 







Melting Tem- 






Melting Tem- 


Lead 


Tin 


perature, 
Degrees F. 


Lead 


Tin 


perature, 
Degrees F. 


14- 


8 


420 


24 


8 


480 


15 


8 


430 


28 


8 


490 


16 


8 


440 


38 


8 


SIO 


17 


8 


45° 


60 


8 


530 


18.5 


8 


460 


96 


8 


550 


20 


8 


470 


200 


8 


560 



These alloys should be carefully made and then run into small 
strips of about \ inch square, being afterward remelted. The 
melting pot should be carefully heated by gas, if possible, and 
the metal should only be heated to such a point that the insertion 
of the tool causes it to set round the steel. When such is the 
case the steel becomes equally heated as the metal melts, and it 
can be allowed to remain some time in the metal before taking 
out and quenching. Another plan is to lay the tools on the cold 
alloy and allow them to remain until it melts, thus permitting the 
steel to gradually warm through, and possibly giving better re- 
sults in the hands of some men. Usually, however, the plunging 
of the tools into the molten metal is the method adopted, and 
provided time is given to allow for the absorption of heat to a 
sufficient depth, this gives good, tough tempering with the requi- 
site hardness. 

The oils for tempering baths specified below are given for the 
sake of convenience only; the statements are based upon the 
findings of thorough experiments. There may, of course, be 
many other oils just as good that have not been tried. 

(1) Walter A. Wood, Boston, Mass., XXX tempering oil; 
as cheap in initial expense as any; good lasting qualities. 



QUENCHING AND TEMPERING 



75 



(2) Strong, Carlisle & Hammond Co., Cleveland, Ohio, Frank- 
fort tempering oil. 

(3) Fish oil, cotton-seed or linseed oil may also be used; in 
many cases these are mixed with high fire and flash test min- 
eral oils. 

The analysis and test results of oils when new (not used) as 
compared with those of oils which have been used such a length 
of time as to render them practically valueless will be found 
interesting. 



Properties of Oil 



Flash point 

Fire test 

Mineral oil, per cent. . . . 
Saponifiable oil, per cent 
Specific gravity 



W. A. Wood 
Tempering Oil 



New 



550 
625 

94 
6 



0.920 



Old 

(Thick) 



475 
55o 

30 

70 
0.950 



Lard and Paraffine Oil 
Mixed (Half and Half) 
Used for Quenching 



New 



400 
475 

25 

75 
0.912 



Old 

(Thick) 



380 

450 
10 
90 
O.925 



Houghton tempering oil: flash point, 595 degrees; fire test, 
685 degrees; specific gravity, 0.900. 

Frankfort tempering oil : fire test, 670 degrees. 

Frankfort quenching oil: fire test, 500 degrees. 

Paraftme oil (Underhay): fire test, 450 degrees; specific grav- 
ity, 0.912. 

Lard oil (Bleecker) : fire test, not determined; specific gravity, 
0.920. 

The great difference in tests and analysis between new and 
used oils should be noted; oils used constantly at high heats 
will gradually lose the " mineral " part of the oil, the more so 
the 'higher the heat used. A tempering bath can therefore be 
prolonged in fife by adding to it now and then new mineral oil. 
To lengthen the life of the bath high heats should be avoided as 
much as possible. 

Practical Tempers for Carbon Steel Tools. — This list of 
tempers was determined by practical shop tests of the tools 



76 HEAT-TREATMENT OF STEEL 

mentioned. A record was kept of the number of pieces machined 
before the tool required sharpening or renewal, and the most 
satisfactory temper adopted. A thermometer was used to de- 
termine degrees of heat. Mutton tallow was used for the bath. 

Degrees F. Class of Tool 

495 to 500 Taps § inch or over, for use on automatic screw machines. 

490 to 495 Taps \ inch or over, for use on screw machines where they 

pass through the work. 

495 to 500 Nut taps \ inch and under. 

515 to 520 Taps \ inch and under, for use on automatic screw machines. 

525 to 530 Thread dies to cut thread close to shoulder. 

500 to 510 Thread dies for general work. 

495 Thread dies for tool steel or steel tube. 

440 to 445 Circular thread chaser for use on lathes. 

525 to 540 Dies for bolt threader threading to shoulder. 

460 to 470 Thread rolling dies. 

430 to 435 Hollow mills (solid type) for roughing on automatic screw 

machine work. 

450 to 455 Hollow mills (solid type) for use on the drill press. 

485 Knurls. 

450 Twist drills for hard service. 

450 Centering tool for automatic screw machine. 

430 Forming tools for automatic screw machine. 

430 to 435 Cut-off tools for automatic screw machine. 

440 to 450 Profile cutters for milling machine. 

430 Formed milling cutters. 

435 to 440 Milling cutters. 

430 to 440 Reamers. 

460 Counterbores and countersinks. 

440 to 450 Fly-cutters for use on the drill press. 

480 Cutters for tube or pipe-cutting machine. 

430 to 440 Dies for heading bicycle spokes. 

430 Punches for heading bicycle spokes. 

430 Backer blocks for spoke drawing dies. 

400 Drawing dies for bicycle spokes. 
460 and 520 Snaps for pneumatic hammers — Harden full length, temper to 

460 degrees, then bring point to 520 degrees. 

Tempering Furnaces. — In tempering furnaces the only really 
important consideration is to insure that the furnace is so built 
as to heat the bath uniformly throughout. It is doubtful if 
there can be found a tempering furnace on the market that will 



QUENCHING AND TEMPERING 



77 



fill this requirement entirely, although many give good results 
in general. It is never safe, however, to let any tools being 
tempered rest against the bottom or sides of the tank, as no 
matter how scientifically the furnace may be built these parts 
are, in most cases, hotter than the fluid itself. It is, of course, 
just as important not to let the thermometer rest against any of 
these parts in order to insure correct readings. After the pieces 
tempered are taken out of the oil bath, they should immediately 
be clipped in a tank of caustic soda (not registering over 8 or 9), 



HOLE FOR THERMOMETER 



18 X 24- 




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Fig. 9. Ordinary Type of Tempering Furnace 

and after that in a tank of hot water. This will remove all oil 
which might adhere to the tools. 

Fig. 9 shows an ordinary type of tempering furnace. In this 
the flame does not strike the walls of the tank directly. The 
tools to be tempered are laid in a basket which is immersed 
in the oil. In Fig. 10 is shown a tempering furnace in which 
means are provided for preventing the tools to be tempered from 
coming in contact with the walls or bottom of the furnace proper. 
The basket holding the tools is immersed in the inner perforated 
oil tank. This same arrangement can, of course, be applied to 
the furnace shown in Fig. 9. 



78 



HEAT-TREATMENT OF STEEL 



HOLE FOR 
THERMOMETER 



Defects in Hardening. — Uneven heating is the cause of most 
of the defects in hardening. Cracks of a circular form, from the 
corners or edges of a tool, indicate uneven heating in hardening. 
Cracks of a vertical nature and dark-colored fissures indicate 
that the steel has been burned and should be put on the scrap 

heap. Tools which have 
hard and soft places have 
been either unevenly 
heated, unevenly cooled, or 
" soaked," a term used to 
indicate prolonged heating. 
A tool not thoroughly 
moved about in the harden- 
ing fluid will show hard 
and soft places, and have a 
tendency to crack. Tools 
which are hardened by 
simply dropping them to 
the bottom of the tank, 
sometimes have soft places, 
owing to contact with the 
floor or sides of the tank. 
They should be thoroughly 
quenched before dropping. 
When a tool appears soft 
and will not harden, it 
probably has been decar- 
bonized on the surface by 
too much heat or by soak- 
ing too long. The surface 
must be removed before 
the tool will harden properly. Tools are sometimes soft because 
the cooling bath is not large enough for the tools being hardened, 
and becomes too warm after a few pieces have been quenched. 

Overheated Steel. — Overheated steel that is not actually 
burned can be partly restored by heating to the proper heat, and 
allowing it to cool slowly in hot ashes or sand; when cold, the 







lURNERS-_ 



YMWM/A 



VMMMMM 




Machinery 



Fig. 10. 



Special Tempering Furnace with 
Perforated Oil Tank 



ANNEALING 79 

steel is hardened again at the proper hardening heat. Tools 
treated in this way are not as good as when treated at the proper 
heat throughout, but they are partially restored, and if the over- 
heating originally took place in forging, the risk of cracking in 
hardening will be lessened by adopting the process mentioned. 
Care should be taken that the tuyere of the forge is well covered 
when heating tool steel; a tool coming in direct contact with 
the air blast will become surface burned, show soft places in 
hardening and wear badly in use. 

Scale on Hardened Steel. — The formation of scale on the 
surface of hardened steel is due to the contact of oxygen with 
the heated steel; hence, to prevent scale, the heated steel must 
not be exposed to the action of the air. When using an oven 
heating furnace, the flame should be so regulated that it is not 
visible in the heating chamber. The heated steel should be ex- 
posed to the air as little as possible, when transferring it from 
the furnace to the quenching bath. An old method of preventing 
scale and retaining a fine finish on dies used in jewelry manu- 
facture, small taps, etc., is as follows: Fill the die impression 
with powdered boracic acid and place near the fire until the acid 
melts; then add a little more acid to insure covering all the sur- 
faces. The die is then hardened in the usual way. If the boracic 
acid does not come entirely off in the quenching bath, immerse 
the work in boiling water. Dies hardened by this method are 
said to be as durable as those heated without the acid. 

Annealing Steel. — The purpose of annealing is not only to 
soften steel for machining, but to remove all strains incident 
to rolling or hammering. A common method of annealing is 
to pack the steel in a cast-iron box containing some material, 
such as powdered charcoal, charred bone, charred leather, slaked 
lime, sand, fireclay, etc. The box and its contents are then 
heated in a furnace to the proper temperature, for a length of 
time depending upon the size of the steel. After heating, the 
box and its contents should be allowed to cool at a rate slow 
enough to prevent any hardening. It is essential, when anneal- 
ing, to exclude the air as completely as possible while the steel 
is hot, to prevent the outside of the steel from becoming oxidized. 



80 HEAT-TREATMENT OF STEEL 

The temperature required for annealing should be slightly 
above the critical point, which varies for different steels. Low- 
carbon steel should be annealed at about 1650 degrees F., and 
high-carbon steel at between 1400 and 1500 degrees F. This 
temperature should be maintained just long enough to heat the 
entire piece evenly throughout. Care should be taken not to 
heat the steel much above the decalescence or hardening point. 
When steel is heated above this temperature, the grain assumes 
a definite size for that particular temperature, the coarseness 
increasing with an increase of temperature. Moreover, if steel 
that has been heated above the critical point is cooled slowly, 
the coarseness of the grain corresponds to the coarseness at the 
maximum temperature; hence, the grain of annealed steel is 
coarser, the higher the temperature to which it is heated above 
the critical point. 

If only a small piece of steel or a single tool is to be annealed, 
this can be done by building up a firebrick box in an ordinary 
blacksmith's fire, placing the tool in it, covering over the top, 
then heating the whole, covering with coke and leaving it to cool 
over night. Another quick method is to heat the steel to a red 
heat, bury it in dry sand, sawdust, lime or hot ashes, and allow 
it to cool. Quick annealing can also be partially effected by 
heating the piece to a dull black-red and plunging it into hot 
water. This method is not to be recommended. 



CHAPTER IV 

HEAT-TREATMENT OF HIGH-SPEED STEEL 

Hardening High-speed Steel. — High-speed steel must be 
heated to a much higher temperature for hardening than carbon 
steel. A temperature of from 1400 to 1600 degrees F. is sufficient 
for carbon steel; high-speed steel requires from 1800 to 2200 de- 
grees F. The usual method of hardening a high-speed steel 
tool, such as a turning or planing tool, is to heat the cutting 
end slowly to a temperature of about 1800 degrees F., and then 
more rapidly to about 2200 degrees F., or until the end is at a 
dazzling white heat and shows signs of melting down. The tool 
point is then cooled either by plunging it in a bath of oil (such 
as linseed or cotton-seed) or by placing the end in a blast of dry 
air. When an oil quenching bath is used, its temperature is 
varied from the room temperature to 350 degrees F., according 
to the steel used. The exact treatment varies for different 
steels and it is advisable to follow the directions given by the 
steel makers. High-speed steel parts that would be injured by 
a temperature high enough to melt the edges are hardened by 
heating slowly to as high a degree as possible and then cooling, 
as described. Formerly, the air blast was recommended by 
most steel makers, but oil is now extensively used. Care should 
be taken to quench the heated steel rapidly after removing from 
the source of heat. The barium-chloride bath has been used 
quite extensively for heating machine-finished, high-speed steel 
tools preparatory to hardening. The barium-chloride forms a 
thin coating on the steel, which is thus protected from oxidation 
while being transferred from the heating bath to the cooling 
bath. Tests have demonstrated, however, that barium-chloride 
baths have certain disadvantages for heating high-speed steel 
preparatory to hardening, because if the steel is heated to the 
required temperature, the surface of the tool is softened to some 

81 



82 HEAT-TREATMENT OF STEEL 

extent. These tests indicate that whenever this salt is used as 
a heating bath, the temperature should not be raised above 
2050 degrees F. When about 0.010 inch is ground from the 
cutting edges of the tools, the objectionable influence of heating 
in barium chloride may be negligible. The influence of barium 
chloride on steel in hardening is treated completely in the chapter 
on " Metallic Salt-bath Electric Hardening Furnaces." 

The oil used may depend on how hard the tool is desired. 
Suppose it is required " dead " hard on the cutting edges; this 
is as hard as it is possible to make it for machine shop use and 
still retain sufficient toughness. To obtain this result, after 
fusing the point or cutting edges, quench quickly in thin lard 
oil, or, for extreme hardness, quench the tool in kerosene oil, 
when about the maximum hardness of this steel may be obtained. 
In using oils, especially kerosene, great care should be taken, or 
the oil may flame and burn the operator. The oil tanks used 
for hardening should be constructed preferably of galvanized 
iron, fitted with close-fitting covers, and provided with a screen 
a few inches from the bottom. 

With regard to the hardening and tempering of specially formed 
tools of high-speed steel, such as milling and gear cutters, twist 
drills, taps, threading dies, reamers, and other tools that do not 
permit of being ground to shape after hardening, and where any 
melting or fusing of the cutting edges must be prevented, the 
method of hardening is as follows: 

A specially arranged muffle furnace heated either by gas or oil 
is employed, consisting of two chambers lined with fireclay, the 
gas and air entering through a series of burners at the back of 
the furnace, and so under control that a temperature up to 2200 
degrees F. may be steadily maintained in the lower chamber, 
while the upper chamber is kept at a much lower temperature. 
Before placing the cutters in the furnace it is advisable to fill up 
the hole and keyways with common fireclay to protect them. 
The cutters are first placed upon the top of the furnace until 
they are warmed through, after which they are placed in the 
upper chamber, and thoroughly and uniformly heated to a tem- 
perature of about 1500 degrees F., or, say, a medium red heat, 



HIGH-SPEED STEEL 83 

when they are transferred into the lower chamber and allowed 
to remain therein until the cutter attains the same heat as the 
furnace itself, viz., about 2200 degrees F. and the cutting edges 
reach a bright yellow heat, having the appearance of a glazed 
or greasy surface. The cutter should then be withdrawn while 
the edges are sharp and uninjured, and revolved before an air 
blast or quenched in oil until the red heat has passed away, and 
then while the cutter is still warm — that is, just permitting of 
its being handled — it should be plunged into a bath of tallow 
at about 200 degrees F. and the temperature of the tallow bath 
then raised to about 520 degrees F., on the attainment of which 
the cutter should be immediately withdrawn and plunged in 
cold oil. 

Of course there are various other ways of tempering, a good 
method being by means of a specially arranged gas-and-air stove 
in which the articles to be tempered are placed, and the stove 
then heated up to a temperature of from 500 degrees F. to 600 
degrees F., when the gas is shut off and the furnace with its 
contents allowed to slowly cool down. 

Very satisfactory results in hardening high-speed steel tools, 
such as cutters, drills, etc., have been obtained by the follow- 
ing method: First pre-heat in an oven- type gas furnace to from 
1300 to 1500 degrees F.; then transfer the steel to another gas 
furnace having a temperature varying from about 2000 to 2200 
degrees F. ; when the steel has attained this temperature, quench 
in a metallic salt bath having a temperature varying from 600 
to 1200 degrees F., depending upon the kind of high-speed steel 
used. The piece to be hardened should be stirred vigorously 
in the bath until it has obtained the temperature of the bath; 
then it is cooled, preferably in the air, and requires no further 
tempering; or it may be put directly into the tempering oil, 
which should be at a temperature anywhere between 100 and 
600 degrees F. The tempering bath is then gradually raised to 
the heat required for tempering. The salt bath for quenching 
should consist of calcium chloride, sodium chloride and potas- 
sium ferro-cyanide, in proportions depending upon the required 
heat. Various kinds of steel require different temperatures for 



84 HEAT-TREATMENT OF STEEL 

the metallic salt bath. After the temper of the tool has been 
drawn in the oil, the work is dipped in a tank of caustic soda, 
and then in hot water. This will remove all oil which might 
adhere to the tools, and is a method that applies to all tools after 
being tempered. 

The method used in the United States Navy yards for harden- 
ing high-speed steel cutting tools is as follows: One oil-heated 
furnace is maintained at a temperature of from 1600 to 1700 de- 
grees F. and another furnace at from 2400 to 2450 degrees F. 
The cutting ends of the tools are pre-heated in the low-temper- 
ature furnace and then transferred to the high-temperature 
furnace. After the tools are removed from the high-heat furnace 
they are cooled by dipping the ends into oil. The oil is agitated 
by compressed air and is cooled and maintained at as even a 
temperature as possible. The tools are cooled in the oil until 
they just show a full black, when they are removed and placed 
on a cooling table. 

The Taylor- White Process. — This process of hardening high- 
speed steel is, in brief, as follows: The first method, commonly 
known as the " high-heat treatment," is effected by heating the 
tool slowly to 1500 degrees F., and then rapidly from that tem- 
perature to just below the melting point, after which the tool is 
quickly cooled below 1550 degrees. At this point, the cooling 
is continued either fast or slow to the temperature of the air. 
It is important to avoid any increase of temperature during the 
cooling period. The second or " low-heat treatment " (temper- 
ing) consists in reheating a tool which has had the high-heat 
treatment to a temperature somewhere between 700 and 1240 
degrees F., preferably in a lead bath, for a period of five minutes. 
The tool is then cooled to the temperature of the air either rapidly 
or slowly. 

Tempering High-speed Steel. — Heavy high-speed steel tools 
having well-supported cutting edges (such as planing or turning 
tools) are commonly used after hardening and grinding, with- 
out tempering. If high-speed steel tools are comparatively 
weak, they are often toughened by tempering to suit the partic- 
ular service required. This is sometimes referred to as " letting 



HIGH-SPEED STEEL 85 

down " the hardness. The steel is heated in lead, oil or in a 
forge fire. A method recommended by several steel makers is 
to cover the steel with clean dry sand and heat to the required 
temperature, as indicated, preferably, by a pyrometer. The sand 
is contained in a metal pan and is heated by a gas or oil burner. 
A general idea of tempering temperatures for high-speed steel 
may be obtained from the following figures: Milling cutters, 
400 degrees F.; threading dies and taps, 490 degrees F.; drills 
and reamers, 440 degrees F. for large sizes, and 460 degrees F. 
for small sizes. All heavy turning and planing tools are left 
untempered. 

Annealing High-speed Steel. — Accurate annealing is of much 
value in bringing the high-speed steel to a state of molecular uni- 
formity, thereby removing internal strains that may have arisen; 
at the same time, annealing renders the steel sufficiently soft to 
enable it to be machined into any desired form for turning tools, 
milling cutters, drills, taps, threading dies, etc. Further advan- 
tage also results from careful annealing by minimizing risks of 
cracking when the steel has to be reheated for hardening. In 
cases of intricately-shaped milling tools having sharp square 
bottom recesses, fine edges or delicate projections, and on which 
unequal expansion and contraction are liable to operate sud- 
denly, annealing has a very beneficial effect toward reducing 
cracking to a minimum. Increased ductility is also imparted by 
annealing and this is especially requisite in tools that have to en- 
counter sudden shocks due to intermittent cutting, such as plan- 
ing and slotting tools, or others suddenly meeting projections or 
irregularities on the work operated on. The annealing of high- 
speed steel is best carried out in muffle furnaces designed for 
heating by radiation only, a temperature of 1400 degrees F. being 
maintained from twelve to eighteen hours according to the sec- 
tion of the bars of steel dealt with. 

A number of other methods are also used for annealing high- 
speed steel. The following method is recommended by one 
of the largest high-speed tool steel manufacturers in America. 
Particular attention is called to the temperatures to which the 
steel to be annealed is to be heated, the time necessary, and also, 



86 HEAT-TREATMENT OF STEEL 

that powdered charcoal is given first, it having the preference 
over fine air-dried lime or powdered mica. 

" In annealing high-speed steel, use an iron box or pipe of 
sufficient size to allow at least one-half inch of packing between 
the pieces of steel to be annealed and the sides of the box or pipe. 
(We call attention here to the fact that it is not necessary that 
each piece of steel to be annealed be kept separate from every 
other piece, but only that the steel be prevented from touching 
the sides of «the annealing pipe or box.) Pack carefully with 
powdered charcoal, fine dry lime or mica. Cover with cap, 
which should be air-tight, but if it is not, then lute on with fire- 
clay. Heat slowly to a full red heat, about 1475 or 1500 de- 
grees F., and hold at this heat from two to eight hours, depending 
on the size of the pieces to be annealed. A piece of 2 by 1 by 8 
inches requires about three hours' time. Cool as slowly as possi- 
ble, and do not expose to the air until cold. A good way is to 
allow the box or pipe to remain in the furnace until cold." 

A method almost identical with the one just described, is given 
in somewhat greater detail by a writer in Machinery. This 
method consists of placing the tool in a wrought-iron or cast- 
iron tube, having space large enough in circumference and length 
to accommodate the work and plenty of packing material. One 
end of the tube can be threaded for a cast-iron cap, which can 
be screwed on and off as desired; the other end can be perma- 
nently fixed on the tube, as it is generally only necessary to use 
the one end. Have a number of |-inch holes drilled in the tube, 
and also a few f^-inch holes in the end caps. The idea of the 
holes in the tube is to procure a vent for letting off the gas which 
is generated when heating the packing material. The end holes 
can be used for a number of test wires, which can be withdrawn 
as the heating progresses, and by this means the operator will 
be able to ascertain the proper heat desired, which should be a 
bright orange or a trifle higher, according to the nature of the 
steel. When the desired heat is reached, regulate the blast 
sufficiently to hold the muffle and its contents at this heat for a 
period long enough to allow the heat to thoroughly penetrate 
the steel. Then the muffle with its contents may be buried in 



HIGH-SPEED STEEL 87 

dry slaked lime or sawdust and ashes, and allowed to cool down 
slowly. If a furnace is used for the heating, allow the muffle to 
stay in until furnace and all cools down, when the steel will 
be found quite easy to work in the machine. Respecting this 
" dead heating " in the furnace, unless the steel is properly 
packed in the muffle in order to exclude the oxygen blown into 
the furnace by the blast, the steel is apt to oxidize and the car- 
bon content thereby becomes lowered, resulting in overannealed 
steel. The packing material used may be any one of several 
kinds now commonly used for other work, but charred leather 
gives the best results, and dry, fine charcoal of a good, clean 
quality is effective. 

Experiments relating to the Annealing of High-speed Steel. — 
A series of experiments was recently made to determine the tem- 
perature to which high-speed steel should be heated for annealing. 
It was found that when this steel was heated to below 1250 de- 
grees F. and slowly cooled, as in annealing, it retained the original 
hardness and brittleness imparted to the steel in forging. When 
heated to between 1250 and 1450 degrees F., the Brinell test 
indicated that the steel was soft, but impact tests proved that 
the steel still retained its original brittleness. However, when 
heated to between 1475 and 1525 degrees F. the steel became 
very soft, it had a beautiful fine-grained fracture, and all of the 
initial brittleness had entirely disappeared. 

In carrying these tests further, to 1600, 1750 and 1850 degrees 
F., it was found that the steel became very soft, but there was 
a gradual increase in brittleness and in the size of the grain, until 
at 1850 degrees F. the steel became again as brittle as unan- 
nealed steel; the fracture at this temperature was dull, dry and 
lifeless, and showed marked decarbonization. Dried air-slaked 
lime was used as a packing medium in making these tests. The 
steel was packed in tubes both ends of which were afterward 
provided with air-tight caps. The decarbonization that took 
place was probably due to the oxygen in the air that had filled 
the intervening spaces between each minute particle of lime, 
before it was packed in the tube, attacking the carbon of the 
steel; this decarbonization would not have taken place if pow- 



88 



HEAT-TREATMENT OF STEEL 



dered charcoal had been used. The latter would have supplied 
all the carbon necessary to combine with any oxygen present 
in the tubes. 

An annealing chart, taken by a Bristol recording pyrometer, 
showing the temperature of one of the annealing furnaces in 
which a well-known grade of high-speed steel is annealed by the 
manufacturer, is shown in Fig. i. The method, which is carried 




Fig. I. Annealing Chart from a Recording Pyrometer showing the Tem- 
perature of a Furnace used for annealing High-speed Steel 

on by this manufacturer day after day, is to first pack the bars 
to be annealed in ten-inch diameter wrought-iron pipes, about 
fourteen feet long, the packing medium being pulverized char- 
coal. Then both ends of the pipes are sealed air-tight with fire- 
clay. The annealing furnaces are fired with coal and are brought 
up to 1500 degrees F. at 7 A. M. At this time the large furnace 
doors are opened and from four to six of the ten-inch pipes, pre- 



HIGH-SPEED STEEL 89 

viously packed with steel and sealed, are rolled into the furnace. 
The doors are then closed and the furnace is continuously fired 
until 5 130 P. M., the temperature being kept as near to 1500 de- 
grees F. as possible. The chart, which shows two days' work, 
will indicate how well this temperature has been maintained. 
At 5 130 P. M. firing is discontinued, all holes that might permit 
the influx of air are closed, and the pipes are permitted to cool 
down slowly with the furnace. It will be seen, by again referring 
to the chart, that there is a gradual drop in temperature from 
the time firing is discontinued until the pipes are taken from 
the furnace the following morning preparatory to beginning 
another day's work. 

The chart also indicates that the temperature of the annealed 
steel, when taken from the furnace, is about 1000 degrees F. 
This temperature is several hundred degrees below the critical 
point, or recalescence point of high-speed steel, this point being 
at about 1350 degrees F., so that the annealed bars can be taken 
from the pipes and permitted to cool to normal temperature 
without further delay, because after cooling to 1000 degrees F. 
they would not again become hard without the application of 
more heat. 

The above method is excellent for annealing high-speed steel 
on a large scale. If it is desired to anneal only a few small pieces 
of this grade of steel rapidly, they can be " water annealed, " 
by a method similar to that used for carbon steels; the temper- 
ature to which the steel is raised, however, is not as high as for 
carbon steel. In water annealing, the piece to be annealed is 
gradually and uniformly heated to 760 degrees F. It is then 
taken from the furnace and plunged into a bath of pure water, 
previously heated to a temperature of 150 degrees F., where it is 
permitted to cool until reduced to the temperature of the bath. 
Afterwards the steel can be drilled, filed, or machined into any 
form with little difficulty. The more care devoted to the heat- 
ing, the better the results will be. To heat rapidly will induce 
internal strains and greatly increase the risk of breakage when 
the pieces are plunged into the water bath. 

Another annealing method which differs considerably from 



go 



HEAT-TREATMENT OF STEEL 
Heat-treatment of High-speed Steel 




* If common forge is used for heating, have a deep fire, plenty of coke and light blast. In gen- 
eral, an oil, gas or coke furnace is preferred. 



HIGH-SPEED STEEL 



91 



Heat-treatment of High-speed Steel 



Kind of 
Steel 


Hardening Heat, Cooling Medium, Temper and Grinding 


Lathe and Planer Tools, etc. 


Taps, Milling Cutters, etc. 


Temperature 


Cooling 
Medium 


Condition 
of Wheel 
used for 
Grinding 


Temperature 


Cooling 
Medium 


Temper 
Required 


Burgess 
No. 4 and 
No. 5 


White weld- 
ing heat 
on point 


Cold air 
blast or 
fish oilt 


Dry wheel 
(grind 
slowly) 














Ark 


Fusing heat 
on point 


Cold air 
blast or 
thin oilt 


Very wet 
wheel 


Light 
yellow 
heat 


out 






Midvale 
Special 
Self- 
hardening 


White heat 


Cold air 

blast 




Dark cherry 
heat; then 
heat in 
white hot 
lead 


Oil, cool 
until 
color dis- 
appears 








Capital 
High- 
speed 


White heat 


Cold air 
blast 


Wet 
sandstone 


Full white 
heat 


Cold air 
blast 


No 
temper 


Heller's 
Alloy 
High- 
speed 


Bright 
yellow 
heat 


Air blast or 
fish oilf 


Wet stone 


Bright 
yellow 
heat* 


Air blast t 


Temper 
seldom 
required 


Blue Chip 


Clear white 
heat on 
point 


Air blast or 
oil 


Dry wheel 


White heat 
just below 
fusing 


Oil 


Straw 
color 


Allen's Air 
Harden- 
ing 


White heat 
on point 


Cold air 
blast or 
water at 
150° F. 


Wet stone 














Bethlehem 
Self-hard- 
ening 


White heat 
on point 


Dry, cold 

air blast 




As hot as 
possible 
without 
fusing 


Oil 








Rex High- 
speed 


Fusing 
heat on 
point 


Cold air 
blast (cool 
until tool 
can be 
held in the 
hand)t 


Wet or dry 


As hot as 
possible 
without 
fusing 


Oilt 


Temper 
if neces- 
sary 


Novo 


Fusing heat 
on point 


Cold air 
blast or 
running 
lard or fish 
oilt 


Wet wheel 
(grind 
slowly) 


Red heat; 
then white 
heat in 
furnace 


Thin fish 
or cotton- 
seed oilt 


According 
to use of 
tool 


Bohler's 
Styrian 
High- 
speed 


White heat 
(not fus- 
ing) 


Cold air 
blast 




White heat, 
just be- 
low fusing 


Fish oil 








Mclnnes 
Extra 
High- 
speed 


Fusing 
heat on 
point 


Air blast or 
fish oil 


Dry stone 


Light 
cherry 
red heat 


Air blast or 
fish oil 







Do not use gas fire. 



Steel must not be dipped in water while hot. 



92 HEAT-TREATMENT OF STEEL 

those outlined on the preceding pages is used by a well-known 
tool manufacturer. While doubt has been expressed as to its 
practicability, it is claimed to give good results. There is only 
one objection; the pieces annealed will scale off somewhat, but 
as the surface is generally machined anyway, this objection is 
— for many classes of work — of no importance. The method 
is as follows: 

Pack the tools to be annealed directly in the oven, one on top 
of the other, the furnace being entirely filled if necessary. Then 
heat the furnace to a temperature not exceeding 1700 or 1750 de- 
grees F. It should not require more than three hours for the 
furnace to reach this heat, which is then maintained for about 
two hours more, or until the temperature of all the tools has been 
raised to that of the furnace itself. (When smaller pieces are to 
be annealed, it is, however, sufficient to maintain the heat for 
about one hour.) Then shut off the heat and at the same time 
close all holes, such as burner and draft holes, as carefully as 
possible, and let the tools cool off in the furnace. This cooling 
takes place very much quicker than when the first-mentioned 
method is used, because the tools are not packed, and, hence, 
there is a saving in time not only in the heating but also in the 
cooling. The greater part of the expense of annealing is thus 
saved on account of the saving in fuel, and the elimination of 
the packing, packing materials and the boxes. 



CHAPTER V 
HEAT-TREATMENT OF ALLOY STEEL 

The rapid development of the automobile industry in America 
has awakened a quick, keen appreciation of the great importance 
of proper heat-treatment of steel. Scientific heat-treatment is 
quite as essential as the quality of the steel. Ordinary steel 
may acquire good physical qualities with proper heat-treatment, 
and the best of steel can be ruined by defective methods. There 
must be thoroughness in the various operations of annealing, 
hardening and tempering, for only treatment carried on with 
care makes uniformity of product possible. 

The difference between ordinary and the best steel is great. 
For example, the elastic limit of ordinary steel is about 40,000 
pounds per square inch, with a reduction in area of, say, 50 per 
cent. Nickel steel properly heat-treated has an elastic limit of 
80,000 to 100,000 pounds per square inch of section, with a re- 
duction in area of 50 per cent, or more. Brittleness does not 
follow proper heat-treatment, the enduring quality being in- 
creased in a greater ratio than the elastic limit. Consequently 
crystallization, fatigue, or whatever we name the cause of break- 
age, is less likely to develop in a properly heat-treated and tem- 
pered material than in an annealed and soft material. This fact, 
discovered in the laboratory and established in actual practice, 
is now commonly accepted by metallurgical experts, notwith- 
standing that it completely overturns previous general belief. 

Another, until recently, commonly accepted belief was that 
strength and stiffness are coordinate, or " the stronger a piece of 
steel, the stiffer it is." To illustrate, it was thought that if one 
piece were twice as strong as another, it would bend only one- 
half as much under a given weight; but actual test has shown 
that a chrome-nickel steel having an elastic limit of 150,000 
pounds or more per square inch of section, bends under a given 

93 



94 HEAT-TREATMENT OF STEEL 

load the same amount as a carbon steel specimen, and this con- 
dition holds true as long as the load is within the elastic limit of 
the weaker material. The elastic limit of a well heat-treated steel 
spring is about 150,000 pounds per square inch, but a spring can 
be made of soft steel. If it is not loaded beyond its elastic 
limit, the spring will return to its original shape after every 
deflection, but the deflection would not be sufficient to make a 
good spring. In fact, it would be hardly noticeable, and the 
spring, of course, would be of little value. Between these ex- 
tremes lie the steels used by the spring makers in the past. 

Not only has the automobile industry forced the spring makers 
to depart from their old materials and methods, but the change 
extends all along the line. Assume that a 0.20 per cent carbon 
steel has been used to advantage for a given design of crank- 
shaft, neither bending nor breaking through long-continued use, 
and that the bearing surfaces are as small in area as can be used 
without heating or excessive wear. A crankshaft of properly 
treated chrome-nickel steel, having an elastic limit four or five 
times as high as the 0.20 per cent carbon steel, would be no 
stiffer, but would have greatly increased life and reliability. 
The steel makers must be prepared to meet these new conditions. 
Sound knowledge of steel has spread fast among intelligent manu- 
facturers; from the knowledge obtained in the laboratories 
established, where all materials are physically and chemically 
tested, they have learned to discriminate in selection. With 
known characteristics, a heat-treatment scientifically conducted 
is sure to bring results that make high-grade steels comparable 
with ordinary steels in about the ratio the latter, in turn, bear 
to cast iron. 

The cost of the materials used in automobile construction 
amounts to about sixty per cent of the total cost of production. 
In view of this fact, the kind of material best suited for the more 
vital parts is highly important. - In the following the composition 
and treatment of some of the most commonly used alloy steels 
are reviewed. The carbon steels used in automobile construc- 
tion are also included, in order to give a complete review of the 
subject. The information given is mainly from a report by the 



ALLOY STEELS 95 

Iron and Steel Division of the Society of Automobile Engineers, 
January, 1912, in which very complete specifications for the 
composition, heat-treatment and properties of various kinds of 
steel were given. Tables relating to the composition, heat-treat- 
ment,; etc., of carbon, nickel, nickel-chromium and chromium- 
vanadium steels are also given. 

The steels specified may be of open hearth, crucible or elec- 
tric manufacture, and must be homogeneous, sound and free 
from physical defects, such as pipes, seams, heavy scale or scabs, 
and surface and internal defects visible to the naked eye. The 
figures given in the tables for physical characteristics or prop- 
erties of the various steels refer to sections common in automobile 
construction, that is, to bars from 1 to i| inch round. The high 
elastic limits can be obtained only on small sections with very 
careful heat- treatment, while the low elastic limits can be expected 
on heavy sections with less refined or severe heat- treatment. 

Carbon Steels. — The 0.10 per cent carbon steel is usually 
known to the trade as soft, basic open-hearth steel, and is com- 
monly used for seamless tubing, pressed steel frames and brake- 
drums, sheet steel brake-bands, etc. It is soft and ductile and 
will stand a great deal of deformation without cracking. In 
its natural or annealed condition it should not be used where a 
great deal of strength is required. The quality of this material, 
however, is improved by cold drawing or rolling. An important 
fact to remember is that this steel when cold drawn or rolled is 
returned to the characteristics of the annealed material by heat- 
ing. This remark also applies to all materials the elastic limit 
of which has been increased by cold working. 

The 0.10 per cent carbon steel in its natural or annealed state 
cannot be easily machined. It will tear badly in turning, thread- 
ing and broaching operations. Heat- treatment has little effect 
upon it, and does not increase its strength but only the toughness. 
The heat- treatment which will produce some stiffness is to quench 
it in oil or water at a temperature of 1500 degrees F. No draw- 
ing is required. This steel will caseharden but is not as suitable 
for this purpose as 0.20 per cent carbon steel. This latter steel 
is often known to the trade as machine steel and is intended pri- 



96 HEAT-TREATMENT OF STEEL 

marily for casehardening. It forges well and machines well, but 
is not suitable for screw machine work. Its particular use is for 
forged, machined and casehardened parts, where strength is not 
of especial importance. It can also be drawn into tubes and 
rolled into cold rolled forms and is a good frame material. It 
can be interchanged with o.io per cent carbon steel for cold 
pressed shapes. 

Heat- treatment of 0.20 per cent carbon steel does not increase 
its strength to any degree, but causes a refinement of the grain 
after forging and increases the toughness; all that is necessary 
is to quench it in oil at 1500 degrees F. The casehardening 
treatment specified in the accompanying tables, is the most 
important treatment for this quality of steel. The heat- treat- 
ment specified as " A " is for parts which do not need to carry a 
great deal of load or withstand shock, but simply must have a 
hard surface. Heat-treatment " B " is for the parts which must 
not only be hard on the surface but also must possess strength, 
as, for example, gears, cam rollers, steering-wheel pivot-pins, etc. 

The 0.30 per cent carbon steel is primarily a structural steel. 
It forges well, machines well, and responds to heat-treatment in 
regard to strength as well as toughness. It is used for such forg- 
ings as axles, driving-shafts, steering pivots and other structural 
parts. This quality of steel is not intended for case-hardening, 
but, by careful treatment, it may be safely casehardened, al- 
though it is used for this purpose only as an emergency. In 
that case, it should be given a double heat-treatment, followed 
by a drawing operation. 

The 0.40 per cent carbon steel is a structural steel of greater 
strength than that previously mentioned. Its uses are more 
limited and generally confined to such parts as demand a high 
degree of strength and a considerable degree of toughness. It 
is commonly used for crankshafts, driving shafts and propeller 
shafts. It has also been used for transmission gears, but is not 
quite hard enough for casehardening, and when casehardened, 
not tough enough to make safe transmission gears. When prop- 
erly annealed it machines well, but is not suitable for screw ma- 
chine work. The 0.50 per cent carbon steel differs but little 



ALLOY STEELS 97 

from that just described, although owing to its higher carbon 
content, it is somewhat harder to machine and also somewhat 
stronger. 

The 0.80 per cent carbon steel is ordinarily known to the 
trade as spring steel, and is generally used for springs of light 
sections. The 0.95 per cent carbon steel is also generally used 
for springs. When properly heat-treated extremely good re- 
sults are possible. The quenching temperature, as specified in 
heat- treatment " F " in the accompanying tables, should, if 
anything, be lower than that specified. Because of the high 
carbon content, the steel is used considerably for heavier types 
of springs. 

Nickel Steel. — Nickel steel is the most generally used of the 
alloy steels. The best quality contains 0.20 to 0.25 per cent 
carbon, 3.50 per cent nickel, 0.50 to 0.80 per cent manganese, 
and not over 0.04 per cent sulphur and phosphorus. With car- 
bon and nickel as given above, the manganese content ought 
never to exceed the limits mentioned. A slightly lower carbon 
content is often used for casehardening purposes, and a higher 
carbon percentage is much used for crankshafts. Nickel steel 
is usually made in the basic open-hearth furnace. It is an excel- 
lent steel for casehardening, and is easier to machine than other 
alloy steels. With regard to the use of all alloy steels it should 
be borne in mind that such steels must be heat-treated and not 
used in the annealed or natural condition. In the latter condi- 
tion they are but slightly superior to plain carbon steels. In the 
heat-treated condition, however, a marked improvement in 
physical characteristics is shown. 

The 0.15 per cent carbon nickel steel, the analysis of which 
is given in the accompanying tables, is suitable for carbonizing 
purposes. Steel of this character properly carbonized and heat- 
treated will produce a part with an exceedingly tough and strong 
core and a hard exterior. This steel can be used for structural 
purposes, but is not especially suitable for this purpose. It is 
intended for casehardened gears and for such other casehardened 
parts as require both strength and hardness. The 0.20 per cent 
carbon nickel steel may be used interchangeably with that just 



98 HEAT-TREATMENT OF STEEL 

described. It is intended primarily for casehardening purposes, 
but may, with suitable heat-treatment, also be used for structural 
parts. The 0.25 per cent carbon nickel steel may also be case- 
hardened successfully and is satisfactory for gears — either of 
the transmission or the rear axle bevel type. The treatment for 
carbonizing must be slightly modified to meet the increase in 
carbon content. It can also be used for many structural parts 
if subjected to heat-treatment " H " or " K." 

The 0.30 per cent carbon nickel steel is primarily used for 
structural parts where strength and toughness are required, for 
example, such parts as axles, crankshafts, driving-shafts and 
transmission shafts. Wide variations as to elastic limits are 
possible by varying the quenching mediums — oil, water or 
brine — and by variations in the drawing temperatures. This 
material may be casehardened, but is not suitable for that pur- 
pose. The 0.35 per cent carbon nickel steel is very similar to 
that just described. 

The 0.40, 0.45 and 0.50 per cent carbon nickel steels are not 
widely used, but are available for certain purposes. A greater 
hardness is obtainable in these steels than in those of the lower 
carbon contents, but as increased brittleness accompanies the 
greater hardness, the treatment given must be modified to meet 
these conditions. For example, the final quenching must be at 
a lower temperature in order to produce the desired toughness 
and other properties. The strength of these steels depends upon 
the heat-treatment and may be controlled closely over a wide 
range. 

Nickel-chromium Steels. — There are three types of nickel- 
chromium steels in common use, known as low, medium and 
high nickel-chromium steels according to the percentages of 
nickel and chromium. Nickel-chromium steels are also made 
either with a high carbon content, and used for oil-hardened gears 
and springs, or with a low carbon content, in which case the steel 
is used for axles, shafts, forged parts, and casehardened gears. 
The high-carbon steel carries about 0.5 per cent of carbon, while 
the low carbon alloy carries 0.25 per cent. The nickel content 
is from 1 to 3.5 per cent, while the chromium varies from 0.30 



ALLOY STEELS 99 

to 1.5 per cent. A special nickel-chrome-tungsten steel is some- 
times used for springs. Nickel-chromium steels possess excellent 
static qualities, but present difficulties in heat-treatment, forging 
and machining. 

Silico-manganese and silico-chromium steels with medium and 
low carbon contents are used to a considerable extent abroad for 
springs and gears. Their relatively low cost favors their use, 
but they do not stand up well when subjected to shocks, and 
are too sensitive to heat-treatment. When handled with great 
care they give good results where the temperatures for the heat- 
treatment can be accurately gaged. Chromium steels with high 
carbon content are used to a considerable extent for balls and 
ball races. 

In general, it may be said that the heat-treatment and prop- 
erties of these steels are much the same as those of the plain 
nickel steels, except that the effects of the heat-treatment are 
somewhat augmented by the presence of chromium. The low 
nickel-chromium steels with carbon contents up to 0.20 per cent 
are intended primarily for casehardening, while those with car- 
bon contents from 0.25 to 0.40 per cent are intended primarily 
for structural purposes. Those with carbon contents from 0.45 
to 0.50 per cent may be used for gears and other structural parts 
where a high degree of strength and hardness is demanded and 
where toughness is not of first importance. 

The medium nickel-chromium steels are of the same composi- 
tion as the low nickel-chromium steels except that they contain 
more nickel and chromium. Their general usage is practically the 
same as already mentioned for the low nickel-chromium steels. 

The high nickel-chromium steels require different heat-treat- 
ments from the other two types mentioned on account of the 
amount of nickel and chromium that they contain. Annealing 
before machining will be found necessary for these steels. The 
higher percentages of nickel and chromium make machining in 
a natural condition difficult. The steels with low carbon con- 
tents are casehardened the same as in the case of low nickel- 
chromium steels, and those with higher carbon contents are used 
for structural parts. In general, these steels are used for parts 



IOO HEAT-TREATMENT OF STEEL 

of an important character, and where unusual strength is de- 
manded. The 0.45 per cent high nickel-chromium steel, for 
example, is used for gears where extreme strength and hardness 
are necessary. The carbon content is sufficiently high to cause 
the material to become hard enough to make a good gear when 
quenched without being casehardened. This steel, however, 
is difficult to forge. During the forging operation it should be 
kept at a high or plastic heat and should not be hammered or 
worked after dropping to ordinary forging temperatures, as 
cracking is liable to follow. On the other hand, too high a tem- 
perature is not advisable, as the steel then becomes red-short 
and breaks. 

Chrome-vanadium Steels. — The chrome- vanadium alloy 
steels are preferably made in the crucible or electric furnace, 
although the open-hearth process is also much used for the pur- 
pose. The open-hearth product, however, is somewhat uncer- 
tain, and while springs of steel made by this process may be 
better than those made from ordinary crucible steel, they can- 
not be compared with springs made of crucible chrome-vana- 
dium steel. For excellent quality the latter product constitutes 
the highest attainment of the steel makers' art, and it seems that 
for springs no material is better suited than this steel. 

Chrome-vanadium steel made with a high carbon content is 
suitable for oil-hardened gears and springs. When made with 
a low carbon content it is used for casehardened gears, and, 
when oil-quenched and annealed, for axles, shafts and steering 
knuckles. When a better material than the best nickel steel is 
needed, the various kinds of chrome-vanadium steel are to be 
recommended. They can be easily forged and can be machined 
more readily than chrome-nickel steels of corresponding carbon 
percentages. 

Chrome-vanadium steels are used for many automobile parts. 
They are used interchangeably with carbon steels, nickel steels 
and nickel-chromium steels. Those qualities which contain 
from 0.15 to 0.20 per cent carbon are intended primarily for case- 
hardened parts, while those of from 0.25 to 0.50 per cent carbon 
are used for structural parts. The 0.25 per cent carbon steel 



ALLOY STEELS IOI 

may be casehardened, but is not suitable for this purpose. The 
0.40 per cent carbon steel is of a very good quality, to be selected 
where a high degree of strength is desired coupled with a moder- 
ate measure of toughness. It is a first-class material for high- 
duty shafts. The 0.45 per cent carbon steel may be used for 
gears and springs. When used for structural parts, if an exceed- 
ingly high strength is desirable, heat-treatment " T " should be 
used instead of treatment " U." The 0.50 per cent carbon 
chrome-vanadium steel is suitable for springs and gears. The 
final drawing temperature must vary with the section of material 
being handled; it must be taken into account, for example, 
whether light spiral springs or heavy flat springs are being heat- 
treated. 

Casehardened vs. Oil-hardened Gears. — Both casehardened 
and oil-hardened gears are largely used in automobile construc- 
tion. As previously mentioned, the chrome- vanadium, chrome- 
nickel and silico-manganese alloys are made with both high and 
low carbon contents. The former contains about 0.45 to 0.60 
per cent carbon and enough other hardening elements so that by 
merely quenching the steel in oil from a bright red heat, surface 
hardening is produced sufficient for ordinary wearing purposes, 
while the hardness does not penetrate deeply into the gear, but 
leaves a tough and strong core. The low carbon alloy steels, 
with about 0.20 per cent carbon, require to be casehardened in 
order to produce a sufficiently hard surface for wearing purposes. 
The observations of many engineers have led them to prefer the 
casehardened gear, the following conclusions being based on the 
results of direct tests on thousands of gears. 

1. The static strength of casehardened gears is equal to that 
of oil-hardened gears, assuming that in both cases steel of the 
same class of appropriate composition has been used, and the 
respective heat-treatments have been equally well and properly 
conducted. 

2. Direct experiments prove that casehardened gears resist 
shocks better than oil-hardened gears. 

3. The casehardened gear resists wear incomparably better, 
although it is perhaps not as silent in action. 



102 HEAT-TREATMENT OF STEEL 

The strong objection to the casehardening is in nine cases out 
of ten doubtless due to the fact that the casehardening operation 
is not properly understood. The depth of the hard case or 
covering, the time and temperature required to produce certain 
results, and the exact control of the conditions, together with 
an accurate knowledge of the material to be treated, are factors 
which enter into successful casehardening. 

To obtain the best results in casehardening ordinary carbon 
steel, the following rules should be observed. Steel containing 
less than 0.12 per cent of carbon, and with a low percentage of 
manganese (less than 0.30 per cent) should be used; the case- 
hardening should be accomplished by a material of a definitely 
known chemical composition, such as a mixture of 60 per cent 
charcoal and 40 per cent barium carbonate, and at a temper- 
ature between 1560 to 1920 degrees F. The higher the temper- 
ature, the more rapid will be the casehardening. After the 
casehardening operation, allow the steel to cool down to about 
1 100 degrees F. Then reheat the work to be casehardened, and 
quench it at 1650 degrees F. This heating and quenching has 
the effect of toughening the center, but the outside will be 
coarse-grained and brittle; therefore heat the material a second 
time to 1470 degrees F. to render the outside non-brittle. 

This procedure is more elaborate than that most commonly 
used, in which pieces are dumped directly from the caseharden- 
ing boxes into water. The process, however, can be somewhat 
modified if one uses a good grade of nickel steel, low in carbon, 
and after having casehardened it at the appropriate temperature, 
permits the material to cool off in the boxes before reheating 
and quenching. In this case, if the material is reheated but once 
to 1470 degrees F. the result will be fully equal to or better than 
those obtained by the most careful annealing and double quench- 
ing of ordinary carbon steels. It is, however, better to give a 
double quenching, as then extraordinary toughness and wearing 
qualities are obtained. 

An ideal way of making a nickel steel gear consists in first 
annealing the blank, then rough machining it approximately 
to size, and then re-annealing before taking the last finishing 



ALLOY STEELS 103 

cut. The gears are then packed in a mixture, as mentioned, 
heated to a temperature of about 1625 to 1650 degrees F., car- 
bonizing to a depth of about g 1 ^ to ^ inch. The gears are then 
permitted to cool in the boxes, are heated to 1500 degrees F., and 
quenched in a hot brine or calcium-chloride solution, and finally 
reheated to 1375 or 1400 degrees F. and quenched in oil. The 
temper need not be drawn. 

Another important point is that of drop forging small parts 
which can also be made from bars in automatic machines. No 
steel is improved by drop forging, although some steels are less 
susceptible to injury than others. In drop forging work, in 
order to give plasticity, the material must be heated to a very 
high temperature. An investigation of drop forging and bar 
cut gears, the former being the product of one of the foremost 
drop forging companies, showed that under static test the bar 
cut gears were fully 25 per cent stronger. 

Necessity of Heat-treatment of Alloy Steels. — While the best 
alloy steels are none too good for most of the parts in automobile 
construction, their qualities will not become pronounced unless 
they receive proper heat-treatment. It is waste of money to 
buy good alloy steels without knowing how to properly treat 
them to bring forth their exceptional qualities. For gaging the 
heat a pyrometer is necessary, but it is too often supposed to 
take care of itself. The best pyrometer of the thermo-couple 
type should be regularly inspected. 

The heat-treatment operations depend upon established scien- 
tific facts, and a lack of appreciation of this causes many 
people to buy high-priced alloy steels from which they get no 
better results than from carbon steel properly handled. As an 
example of the effect of heat-treatment may be mentioned a 
chrome steel which in its rolled condition had an elastic limit of 
158,000 pounds, 5 per cent elongation, and 9.4 per cent reduction 
in area. The same steel, oil tempered and annealed, had an 
elastic limit of 153,000 pounds, 14 per cent elongation and 52 per 
cent reduction in area. In other words, the material was trans- 
formed from brittle to tough without appreciably affecting its 
elastic limit. 



io4 



HEAT-TREATMENT OF STEEL 



Composition, Heat-treatment and Properties 

(Information given is based on, and condensed from, the report of the Iron 



Kind of Steel and 

Nominal Carbon 

Content 


Carbon, 
Per cent 


Manganese, 
Per cent 


Phos- 
phorus,* 
Per cent 


Sulphur,* 
Per cent 


Nickel, 
Per cent 


w 
CD 
XIX 

o 

o 

w 

T QJ 
CD 
+^> 

CO 

Ij 
^4 

CJ 

g 


o.io Carbon 

o . 20 Carbon 

0.30 Carbon 
. 40 Carbon 
. 50 Carbon 
. 80 Carbon 
0.95 Carbon 


O.05-O.15 
(o.io) 

O.15-O.25 
(0. 20) 

0.25-0.35 
(0.30) 

O.35-0.45 
(0.40) 

0.45-0.55 
(0.50) 

O.75-O.90 

(0.80) 

O . 90-I . 05 
(o.95) 


O.30-O.60 
(o.45) 

O . 50-0 . 80 
(0.65) 

O . 50-O . 80 
(0.65) 

. 50-O . 80 
(0.65) 

. 50-0 . 80 
(0.65) 

O.25-O.50 

(o.35) 
O.25-0.50 

(o.35) 


O.04 

O.04 

O.04 
O.04 
O.04 
O.04 
O.04 


O.04 

O.04 

O.04 
O.04 
0.04 
O.04 
0.04 




0.15 Carbon 
Nickel 

. 20 Carbon 
Nickel 

0.25 Carbon 
Nickel 

0.30 Carbon 

Nickel 

0.35 Carbon 
Nickel 

0.40 Carbon 

Nickel 

0.45 Carbon 
Nickel 

. 50 Carbon 
Nickel 


0.10-0.20 

(0.15) 

O.15-O.25 
(0. 20) 

O . 20-0 . 30 
(0.25) 

O.25-O.35 
(0.30) 

O.30-O.40 
(o.35) 

0.35-0.45 
(0.40) 

O . 40-O . 50 

(o.45) 

0.45-0.55 
(0.50) 


O . 50-O . 80 
(0.65) 

O . 50-0 . 80 

(0.65) 

O . 50-0 . 80 
(0.65) 

O . 50-0 . 80 

(0.65) 

. 50-O . 80 

(0.65) 

O . 50-0 . 80 

(0.65) 

O . 50-0 . 80 
(0.65) 

O . 50-0 . 80 
(0.65) 


O.04 
O.04 
O.04 
O.04 
O.04 
O.04 
O.04 
O.04 


O.04 
O.04 
O.04 
0.04 
O.04 
O.04 
O.04 
O.04 


3-25-3-75 
(3-50) 

3 • 25-3 • 75 
(3.5o) 

3 • 25-3 • 75 
(3-5o) 

3-25-3-75 
(3-5o) 

3-25-3-75 
(3.5o) 

3 • 25-3 • 75 
(3-5o) 

3 • 25-3 • 75 
(3.5o) 

3-25-3-75 
(3.5o) 



* Phosphorus and sulphur not to exceed values given. 
Values within parentheses are preferred percentages. 



ALLOY STEELS 



I05 



of Carbon and Special Alloy Steels — 1 

and Steel Division of the Society of Automobile Engineers, January, 1912.) 



Carbon 
Con- 
tent 


Heat- 
treatment f 


Elastic Limit, 
Pounds per Square Inch 


Reduction in 

Area, 

Per cent 


Elongation in 
2 inches, 
Per cent 


Annealed 


Heat-treated 


An- 
nealed 


Heat- 
treated 


An- 
nealed 


Heat- 
treated 


O. IO 

0.20 

O.30 

O.40 

O.50 
O.80 
0-95 


Quench at 
1500 F. 

A ot B 

C ot D 

E 

E 

F 
F 


28,000- 
36,000 

30,000- 
40,000 

35.000- 
45,000 

40,000- 
50,000 

45,000- 
60,000 




55-65 
45-60 

40-55 
40-50 
30-40 


15-60 
30-60 
25-55 
I5-50 


30-40 
25-35 
20-30 

20-25 
15-20 


15-35 
IO-30 

5-25 
5-20 


40,000-75,000 
40,000-80,000 
45,000-100,000 

50,000-110,000 

90,000-160,000 
90,000-160,000 


O.I5 
0.20 
O.25 
O.30 

0-35 
O.40 

0.45 
O.50 


G 
HotK 

G 
HotK 

G 
HotK 

HotK 
HotK 
Hot K 
Hot K 
Hot K 


35.000- 
45,000 

40,000- 
50,000 

40,000- 
50,000 

45.000- 
55.000 

45.000- 
55.000 

55.000- 
70,000 

55.000- 
70,000 

55.000- 
70,000 


(H or K) 40,000 
-80,000 

{H or Ky 50,000 
-125,000 

{H or K) 60,000 
-130,000 

65,000-150,000 
65,000-160,000 
70,000-200,000 
70,000-200,000 
70,000-200,000 


45-65 
40-65 
40-60 
35-55 
35-55 
30-50 
30-50 
30-50 


40-65 
40-65 
30-60 
25-55 
25-55 
15-55 
15-55 
15-55 


25-35 
20-30 
20-30 
15-25 
15-25 
15-25 
15-25 
15-25 


15-35 

IO-25 

IO-25 

IO-25 

IO-25 

5-20 

5-20 

5-20 



f See Table 4 for detailed description of heat-treatment. 



io6 



HEAT-TREATMENT OF STEEL 



Composition, Heat-treatment and Properties 

(Information given is based on, and condensed from, the report of the Iron 



Kind of Steel and 

Nominal Carbon 

Content 


Carbon, 
Per cent 


Manganese, 
Per cent 


Phos- 
phorus,* 
Per cent 


Sulphur,* 
Per cent 


Nickel, 
Per cent 


in 
% 

O) 

w 

6 
B 

' a 

o 

O 

o 

o 

ta 

13 

<D 
+-> 
CZJ 

a 
a 

o 
o 

1 

15 

o 

2 

a 

2 


0.15 Carbon 
Low Ni-Cr 

. 20 Carbon 
Low Ni-Cr 

0.25 Carbon 
Low Ni-Cr 

0.30 Carbon 
Low Ni-Cr 

0.35 Carbon 
Low Ni-Cr 

0.40 Carbon 
Low Ni-Cr 

0.45 Carbon 
Low Ni-Cr 

0.50 Carbon 
Low Ni-Cr 


O.IO-O.20 
(O.I5) 

O.15-O.25 
(0.20) 

O.20-O.30 

(0.25) 

O.25-O.35 
(0.30) 

O . 30-O . 40 

(o.35) 

0.35-0.45 
(0.40) 

O . 40-0 . 50 

(o.45) 

0.45-0.55 
(0.50) 


O.50-0.80 
(0.65) 

O . 50-0 . 80 
(0.65) 

O.50-0.80 
(0.65) 

O . 50-O . 80 

(0.65) 

O.50-O.80 
(0.65) 

O . 50-0 . 80 

(0.65) 

O . 50-0 . 80 
(0.65) 

O . 50-O . 80 

(0.65) 


0.04 
0.04 
0.04 
0.04 
0.04 
0.04 
0.04 
0.04 


O.04 
O.04 
O.04 
O.04 
O.04 
O.04 
O.04 
O.04 


i . 00-1 . 50 

I. OO-I.50 

i . 00-1 . 50 
i . 00-1 . 50 
1 . 00-1 . 50 
I. 00-1.50 
1 . 00-1 . 50 
1 . 00-1 . 50 


0.15 Carbon 
Med. Ni-Cr 

. 20 Carbon 
Med. Ni-Cr 

0. 25 Carbon 
Med. Ni-Cr 

0.30 Carbon 

Med. Ni-Cr 

0.35 Carbon 
Med Ni-Cr 

. 40 Carbon 
Med. Ni-Cr 

0.45 Carbon 
Med. Ni-Cr 


0.10-0. 20 

(0.15) 

0.15-0.25 

(0.20) 
. 20-0 . 30 

(0.25) 

0.25-0.35 
(0.30) 

. 30-0 . 40 

(0.35) 

0.35-0.45 
(0.40) 

. 40-0 . 50 

(0.45) 


O . 30-O . 60 
(o.45) 

O . 30-O . 60 
(o.45) 

O.30-O.60 

(o.45) 

O . 30-O . 60 

(o.45) 

O . 30-O . 60 

(o.45) 

O.30-O.60 

(o.45) 
O.30-O.60 

(o.45) 


0.04 

0.04 

0.04 

0.04 

0.04 

0.04 
0.04 


O.04 

O.04 

O.04 

O.04 

O.04 

O.04 
O.04 


1.50-2.00 

(1.75) 

I . 50-2 . 00 
(i-75) 

1.50-2.00 
(1-75) 

1 . 50-2 . 00 
(1-75) 

1 . 50-2 . 00 
(1-75) 

1 . 50-2 . 00 

(i.75) 
1 . 50-2 . 00 

(i.75) 



Phosphorus and sulphur not to exceed values given. 
Values within parentheses are preferred percentages. 



ALLOY STEELS 



107 



of Carbon and Special Alloy Steels — 2 

and Steel Division of the Society of Automobile Engineers, January, 191 2) 



Car- 
bon. 
Con- 
tent 

O.I5 
0.20 
O.25 
O.30 

0.35 
O.40 

0.45 
O.50 

O.I5 
0.20 
O.25 
O.30 

0.35 
O.40 
0.45 


Chromium, 
Per cent 


Heat- 
treat- 
ment t 


Elastic Limit, 
Pounds per Square Inch 


Reduction in 

Area, 

Per cent 


Elongation in 
2 Inches, 
Per cent 


Annealed 


Heat-treated 


An- 
nealed 


Heat- 
treated 


An- 
nealed 


Heat- 
treated 


O.30-0.75 
O.30-O.75 
O.30-O.75 
O.30-O.75 
0.30-O.75 
O.30-O.75 
O.30-O.75 
O.30-O.75 


G 

Hot K 

G 
Hot K 

Hot K 
HotK 
HotK 
Hot K 

K 

K 


30,000- 
40,000 

30,000- 
40,000 

40,000- 
55.000 

40,000- 
55.000 

45.000- 
60,000 

45,000- 
60,000 

55.000- 

70,000 

55.000- 
70,000 


{H or K) 
40,000-100,000 

{H or K) 
40,000-100,000 

50,000-125,000 
50,000-125,000 
55,000-150,000 
55,000-150,000 
60,000-175,000 
60,000-175,000 


40-55 
40-55 
35-50 
35-50 
30-45 
30-45 
30-50 
30-50 


40-65 
40-65 
25-55 
25-55 
25-50 
25-50 
20-45 
20-45 


25-35 
25-35 
20-30 
20-30 
15-25 
15-25 
15-25 
15-25 


15-25 

15-25 

IO-25 

IO-25 

5-20 

5-20 

5-15 

5-15 


O.75-1.25 
(o.75) 

O.75-I.25 
(1.00) 

0.75-1.25 

(1.00) 

0.75-1.25 

(1.00) 

0.75-1.25 

(1.00) 

0.75-1.25 

(1.00) 

0.75-1-25 

(1.00) 


G 
Hot K 

G 
HotK 

Hot K 

HotK 

HotK 

Hot K 
K 


35,000- 
45.000 

35,000- 

45,000 

40,000- 
50,000 

45,000- 
50,000 

45,000- 
55.000 

50,000- 
60,000 

55,000- 
65,000 


(H or K) 
45,000-110,000 

(H or K) 
45,000-110,000 

55,000-130,000 

55,000-150,000 

60,000-160,000 

60,000-175,000 
100,000-200,000 


45-55 
45-55 
40-55 
35-50 
35-50 

35-45 
35-45 


35-65 
35-65 
30-60 

30-55 
30-55 
25-50 
20-35 


20-30 

20-30 

20-30 

15-25 

15-25 

15-25 
15-25 


IO-25 
IO-25 

IO-25 

IO-25 

5-20 

5-20 

0-I5 



t See Table 4 for detailed description of heat -treatment. 



io8 



HEAT-TREATMENT OF STEEL 



Composition, Heat-treatment and Properties 

(Information given is based on, and condensed from, the report of the Iron 



Kind of Steel and 

Nominal Carbon 

Content 


Carbon, 
Per cent 


Manganese, 
Per cent 


Phos- 
phorus,* 
Per 
cent 


Sul- 
phur,* 
Per 
cent 


Nickel, 
Per cent 


Chromium, 
Per cent 


w 

'a; 
<v 
*-> 

CO 

B 

1 

u 
o 

% 
o 

2 

B 

w 

<v 
CO 

B 

U 
o3 
> 

a 

o 
u 
xi 
O 


0.15 Carbon 
High Ni-Cr 

. 20 Carbon 
High Ni-Cr 

0.25 Carbon 
High Ni-Cr 

0.30 Carbon 
High Ni-Cr 

0.35 Carbon 
High Ni-Cr 

0.40 Carbon 
High Ni-Cr 

0.45 Carbon 
High Ni-Cr 


. 10-0 . 20 
(O.I5) 

0.15-0.25 
(0 . 20) 

0.20-0.30 

(0.25) 

0.25-0. 35 
(0.30) 

. 30-0 . 40 
(o-35) 

0-35-0-45 
(0.40) 

. 40-0 . 50 

(o.45) 


O . 30-0 . 60 
(o.45) 

O . 30-O . 60 

(o.45) 

O . 30-0 . 60 
(o.45) 

. 30-0 . 60 
(o.45) 

O.30-O.60 
(o.45) 

O . 30-0 . 60 

(o.45) 

O . 30-0 . 60 

(o.45) 


O.04 
O.04 
O.04 
O.04 
O.04 
O.04 
0.04 


O.04 
0.04 
O.04 
O.04 
0.04 
0.04 
0.04 


3-25-3-75 
(3-50) 

3 • 25-3 • 75 
(3.5o) 

3-25-3-75 
(3.5o) 

3 • 25-3 • 75 
(3.5o) 

3-25-3-75 
(3-5o) 

3 • 25-3 • 75 
(3-5o) 

3-25-3-75 
(3-5o) 


I. 25-I. 75 

(I-SO) 

I-25-I-75 
(I-SO) 

I-25-I-75 
(I-SO) 

i. 25-1. 75 
(1.50) 

1-25-1-75 
(i- So) 

1-25-1-75 
(1.50) 

1-25-1.75 
(1.50) 


0.15 Carbon 
Cr-Va 

. 20 Carbon 
Cr-Va 

0.25 Carbon 
Cr-Va 

0.30 Carbon 
Cr-Va 

0.35 Carbon 
Cr-Va 

0.40 Carbon 
Cr-Va 

0.45 Carbon 
Cr-Va 

. 50 Carbon 
Cr-Va 


. 10-0 . 20 

(0.15) 

0.15-0.25 

(O- 2 ©), 

O.2O-O.3O 

(0.25) 

O.25-O.35 
(0.30) 

O . 3O-O . 40 

(0.35) 

0.35-0.45 
(0.40) 

O . 4O-O . 50 

(0.45) 

0-45-0-5S 
(0.50) 


O . 50-0 . 80 
(0.65) 

O . 50-0 . 80 
(0.65) 

O . 50-0 . 80 
(0.65) 

O . 50-O . 80 
(0.65) 

O . 50-O . 80 
(0.65) 

O.50-O.80 
(0.65) 

O . 50-0 . 80 
. (0.65) 

O . 50-O . 80 
(0.65) 


O.04 
O.04 
O.04 
O.04 
O.04 
0.04 
O.04 
0.04 


O.04 
O.04 
O.04 
O.04 
O.04 
O.04 
0.04 
O.04 




. 70-1 . 10 
(0.90) 

0.70-1. 10 
(0.90) 

. 70-1 . 10 
(0.90) 

0.70-1. 10 
(0.90) 

. 70-1 . 10 
(0.90) 

. 70-1 . 10 
(0.90) 

. 70-1 . 10 
(0.90) 

0.70-1. 10 
(0.90) 



* Phosphorus and sulphur not to exceed values given. 
Values within parentheses are preferred percentages. 



ALLOY STEELS 



109 



of Carbon and Special Alloy Steels — 3 

and Steel Division of the Society of Automobile Engineers, January, 191 2) 



Carbon 
Con- 
tent 


Vana- 
dium, t 
Per cent 


Heat- 
treatment t 


Elastic Limit, 

Pounds per Square 

Inch 


Reduction in 

Area, 

Per cent 


Elongation 

in 2 inches, 

Per cent 


Annealed 


Heat- 
treated 


An- 
nealed 


Heat- 
treated 


An- 
nealed 


Heat- 
treated 


O.I5 
O.20 
O.25 
O.30 

0.35 
O.40 

0.45 




L 

L 

Mot P 

Mot P 

M OT P 

M or P 
P 

Q 


40,000- 
50,000 

40,000- 
50,000 

40,000- 

50, JOO 

45,000- 
55.000 

45.000- 
55.000 

50,000- 
60,000 

50,000- 
60,000 


(M or P) 
50,000- 
125,000 

60,000- 
140,000 

60,000- 
175,000 

60,000- 

175,000 

65,000- 

200,000 

150,000- 
250,000 


45-60 
45-60 
45-60 
40-55 
40-55 
40-50 
40-50 


30-65 
30-65 
30-60 
30-60 

20-50 
15-25 


20-25 
20-25 
20-25 
15-25 
15-25 
15-25 
15-25 


5-20 
5-20 
5-20 
5-20 
2-15 
2-15 


O.I5 
0.20 
O.25 
O.30 

0.35 
O.40 

0.45 
O.50 


O.I2 
(0.18) 

O.I2 

(0.18) 

O.I2 
(0.18) 

O.I2 
(0.18) 

O.I2 
(O.I8) 

O.I2 
(0.18) 

O.I2 
(0.18) 

O.I2 
(0.18) 


Sot T 
T 
T 
T 
T 
T 
U 
U 


35.000- 
45.000 

40,000- 
50,000 

40,000- 
50,000 

45.000- 
55.000 

45.000- 
55.000 

50,000- 
60,000 

55.000- 

65,000 

60,000- 
70,000 


CO 

50,000- 
90,000 

55.000- 

100,000 

55.000- 

100,000 

60,000- 

150,000 

60,000- 
150,000 

65,000- 
175,000 

150,000- 
200,000 

150,000- 
225,000 


50-70 
50-65 
50-65 
50-60 
50-60 
45-55 
40-55 
35-50 


40-70 
45-65 
45-65 
25-55 
25-55 
I5-50 
IO-25 

15-35 


25-30 
20-30 
20-30 
20-25 
20-25 
15-25 
15-25 
15-20 


IO-25 
IO-25 
IO-25 
5-15 
5-15 
2-15 
2-IO 
2-IO 



Vanadium content not to be less than 0.12 per cent. 
See Table 4 for detailed description of heat -treatment. 



no 



HEAT-TREATMENT OF STEEL 



Heat-treatment of Carbon and Alloy Steels — 4 

Heat- treatments specified for various steels listed in preceding Tables i, 

2 and 3 



Heat-treatment A 


Heat-treatment F 


After forging or machining: 


After shaping or coiling: 


1. Carbonize at a temperature 


1. Heat to i425°-i475° F. 


between 1600 F. and 1750 


2. Quench in oil. 


F. (i65o°-i7oo° F. desired). 


3. Reheat to 4oo°-8oo° F. in ac- 


2. Cool slowly or quench. 


cordance with degree of 


3. Reheat to 1450 °-i 500 ° F. and 


temper desired, and cool 


quench. 


slowly. 




Heat-treatment G 


Heat-treatment B 


After forging or machining: 


After forging or machining: 


1. Carbonize at a temperature 


1. Carbonize at a temperature 


between 1600 F. and 1750 


between 1600 F. and 1750 


F. (i65o°-i7oo° F. desired). 


F. (i65o°-i7oo° F. desired). 


2. Cool slowly in carbonizing 


2. Cool slowly in the carboniz- 


material. 


ing mixture. 


3. Reheat to i45o°-i525° F. 


3. Reheat to i5oo°-i55o° F\ 


4. Quench. 


4. Quench. 


5. Reheat to i3oo°-i4oo° F. 


5. Reheat to 1400 °-i 450 ° F. 


6. Quench. 


6. Quench. 


7. Reheat to a temperature from 


7. Draw in hot oil at a tempera- 


25o°-5oo° F. (in accordance 


ture which may vary from 


with the necessities of the 


300 to 450 F., depending 


case) and cool slowly. 


upon the degree of hardness 


Heat-treatment H 


desired. 


After forging or machining: 


Heat-treatment C 


1. Heat to i5oo°-i55o° F. 

2. Quench. 

3. Reheat to 6oo°-i2oo° F. and 


After forging or machining: 


1. Heat to i475°-i525° F. 


cool slowly. 


2. Quench. 


Heat-treatment K 


3. Reheat to 6oo°-i20o° F. and 
cool slowly. 


After forging or machining: 
1. Heat to i5oo°-i55o° F. 


Heat-treatment D 


2. Quench. 

3. Reheat to i3oo°-i4oo° F. 


After forging or machining: 


4. Quench. 


1. Heat to i5oo°-i55o° F. 


5. Reheat to 6oo°-i2oo° F. and 


2. Quench. 


cool slowly. 


3. Reheat to i4oo°-i45o° F. 


Heat-treatment L 


4. Quench. 

5. Reheat to 6oo°-i2oo° F. and 

cool slowly. 


After forging or machining: 

1. Carbonize at a temperature 
between 1600 ° F. and 1750 


Heat-treatment E 


F. (1650 °-i 700 °F. desired). 
2. Cool slowly in the carboniz- 


After forging or machining: 


ing mixture. 


1. Heat to i5oo°-i55o° F. 


3. Reheat to 1400 °-i 500 ° F. 


2. Cool slowly. 


4. Quench. 


3. Reheat to 1400 °-i 450 ° F. 


5. Reheat to i30o°-i4oo° F. 


4. Quench. 


6. Quench. 


5. Reheat to 6oo°-i2oo° F. and 


7. Reheat to 25o°-5oo° F. and 


cool slowly. 


cool slowly. 



ALLOY STEELS 



III 



Heat-treatment of Carbon and Alloy Steels — 4 {Continued) 

Heat- treatments specified for various steels listed in preceding Tables 1, 

2 and 3 



II eat- treatment M 


Heat-treatment S 


After forging or machining: 


After forging or machining: 


1. Heat to i45o°-i5oo° F. 


1. Carbonize at a temperature 


2. Quench. 


between 1600 F. and 1750 


3. Reheat to a temperature be- 
tween 500° F. and 1250 F. 


F. (i65o°-i7oo° F. desired). 


2. Cool slowly in the carboniz- 


and cool slowly. 


ing mixture. 
3. Reheat to 1600 °-i 700 ° F. 


Heat-treatment P 


4. Quench. 




5. Reheat to i475°-i55o° F. 


After forging or machining: 


6. Quench. 


1. Heat to i45o°-i5oo° F. 


7. Reheat to 25o°-s5o° F. and 


2. Quench. 


cool slowly. 


3. Reheat to i 3 75°-i425 F. 




4. Quench. 


Heat-treatment T 


5. Reheat to a temperature be- 
tween 500 ° F. and 1250 F. 
and cool slowly. 


After forging or machining: 

1. Heat to 1600 °-i 700 ° F. 

2. Quench. 


Heat-treatment Q 


3. Reheat to some temperature 
between 500 °F. and 1300 °F. 


After forging: 


and cool slowly. 


1. Heat to 1475 °-i525°F. (Hold 
at this temperature one- 


Heat-treatment U 


half hour, to insure thor- 


After forging: 


ough heating.) 


1. Heat to 1525 °-i6oo° F. (Hold 


2. Cool slowly. 


for about one-half hour.) 


3. Reheat to 1450 °-i 500 ° F. 


2. Cool slowly. 


4. Quench. 


3. Reheat to 1650°-!: 700 ° F. 


5. Reheat to 25o°-55o° F. and 


4. Quench. 


cool slowly. 


5. Reheat to 35o°-55o° F. and 




cool slowly. 



CHAPTER VI 

HEAT-TREATMENT OF STEEL BY THE ELECTRIC 
FURNACE 

In properly managed shops, the heat-treatment of steel is 
today receiving thorough attention. To produce a tool of such 
high quality that it will give several times the service of a tool 
that has not been properly heat-treated, is an important factor 
in shop economy. To accomplish this result it is necessary to 
know the laws governing the hardening of steel. If we clearly 
understand the causes underlying the changes which take place 
when steel is subjected to various heat-treatments, we have the 
basis for a positive control of the quality of the finished product. 
" Electric heat " is a new and important means to this end. 

We heat and quench a tool because we want it to be harder. 
In every case the object of this treatment is to change, in some 
degree, certain of the physical properties of the steel. The 
effect of heat upon a piece of steel depends on the nature of the 
steel — that is, upon its composition, its form or external shape, 
and its internal structure. The raising of the temperature of 
the piece sufficiently will produce a change in the form, so as to 
increase its volume by causing a lengthening in some directions. 
In the structure of the steel itself this may introduce mechanical 
strains in the fibers. Mostly, however, these are but temporary 
changes, and, with proper heat-treatment, they disappear when 
the piece is again cooled. 

Changes in the chemical arrangement of the elements compos- 
ing the steel are produced when the temperature of the piece is 
raised to a sufficient degree. These are the changes that are 
effective in hardening and tempering. If the piece, once heated 
to a sufficient temperature to produce hardening, is allowed to 
cool very slowly, these <l changes " of chemical arrangements 
revert to their original condition; but if the piece is cooled 



ELECTRIC HEATING FURNACES 113 

quickly — quenched — immediately upon removing it from the 
source of heat, the changes are made permanent. 

For steels of different composition, that is, made up of either 
different elements or of different proportions of the same ele- 
ments (iron, carbon, etc.), there are, as explained in Chapter I, 
different critical temperatures at which these changes take place. 
Corresponding differences in the heat-treatment are, therefore, 
necessary to produce the best results. Even two pieces of the 
same steel which vary greatly in their form must be treated 
differently. This is true also of two pieces of steel the composi- 
tion and form of which may be identical, but the ultimate use 
of which may be different. 

Examples of all three of these conditions occur constantly in 
shop practice. The intelligent hardener knows that a compli- 
cated die must be handled differently from a straight lathe tool, 
and a shaper tool for soft metal need not be as hard as one for 
hard metal, even though all of these pieces be made from the 
same steel. The next step ahead is to treat different kinds of 
steels according to their particular requirements. Various high- 
speed steels, high-carbon steels, and low-carbon steels are given 
individual treatment. Each is subjected to the conditions of 
heating and subsequent handling that will bring out the maxi- 
mum of its useful characteristics. To meet the wide range of 
requirements and to avoid the losses of tools spoiled in harden- 
ing, there are three factors in the heating of steel which should 
be observed: First, the quality of the heat; second, its uniform- 
ity; and third, its degree. 

Under ideal conditions the steel would be subjected to a heat 
effect only, as this alone is necessary to produce the desired 
changes. In practice, however, it is difficult to produce heat 
of the necessary intensity without having the quality of it im- 
paired by the presence of flames or oxidizing gases and sulphur 
and other injurious fumes. Freeing the heat from such attendant 
defects would obviously greatly improve the quality of the fin- 
ished product; also the more uniformly heat can be applied to 
all parts of the piece, the more uniform will be the hardening or 
tempering. These two factors are positive and constant in their 



114 HEAT-TREATMENT OF STEEL 

desirability for all ordinary work. The third factor — the de- 
gree of heat — is the variable quantity. It is this factor, the 
temperature, that is chiefly manipulated to meet the require- 
ments of different steels and of the same steels for different 
purposes. While other conditions have a certain influence, the 
temperature is the controlling factor in all heat- treatment. The 
evil effects of too high a temperature — the common failing — 
are well understood. 

The length of time during which the steel is actually heated is 
an important point closely connected to that of the temperature. 
With a heating chamber of sufficient size to supply the necessary 
heat to the piece, the internal change in the steel that results 
in hardening can be effected, in general, in one of two ways; 
either the piece may be heated for a short time at a relatively 
high temperature, or for a longer interval at a lower temper- 
ature. Both may produce hardness. Fractures of small pieces 
of steel which after pre-heating were heated for but thirty seconds 
at a temperature 300 or 400 degrees higher than the hardening 
point, often compare very favorably with similar fractures of 
the same steel heated for four or five minutes at a temperature 
but little above the critical point. Thus the quantity of heat 
absorbed by the piece being treated is seen to be practically a 
product of the temperature of the heating chamber and the 
time the piece is left in it. In other words, the time of heating 
varies inversely as the temperature of the chamber in which the 
piece is heated. This still further emphasizes that " temper- 
ature should be the controlling factor," because, of the two ex- 
tremes, the ordinary dangers of burning the steel on account of 
too high a temperature, and of causing it to crack due to too 
rapid, irregular heating, are far greater than those of " over- 
soaking " at lower temperature. To heat at the lower temper- 
ature is plainly the safe course, due both to its cutting down the 
grave danger of over-heating and to the greater uniformity with 
which heat is absorbed by the piece. 

It is clear, then, that the heat-treatment of a particular steel 
can be greatly improved by definitely knowing beforehand the 
correct temperature at which it should be hardened. Also, when 



ELECTRIC HEATING FURNACES 115 

a large number of tools of approximately uniform sizes and shapes 
are being handled, the time necessary for proper heat absorption 
should first be determined, using an experimental tool that 
represents a fair sample. It will be found that the time ele- 
ment varies practically in proportion to the thickness of the 
steel. From the furnace standpoint, therefore, the accurate 
and flexible control of the temperature is a most important con- 
sideration. A positive means, such as a pyrometer, for indi- 
cating at any time just what the temperature is, becomes, of 
course, an incidental requirement. 

Until recently, the only known way of producing heat of the 
required intensity was by combustion — the burning of some 
fuel. The attendant disadvantages of this are well known. The 
crude open coal forge is capable of heating the steel, but leaves 
much to be desired as regards the quality of the heat, its uni- 
formity, and the temperature control. In order to produce heat 
at all, the carbon in the coal must be combined with the oxygen 
of the air, and a strongly oxidizing flame is unavoidable. The 
steel exposed to this action, or to the inevitable results of it, suffers 
accordingly. The coke-burning furnace offered some improve- 
ments, but only in detail. Now there are highly-perfected fur- 
naces for burning oil and gas, and some of these offer still further 
advances, but the principle at the basis of all of these is the same 
— there must be a " burning " process to produce the heat; 
oxidation must be present with all fuel-combustion furnaces. 

Through what means, then, may we obtain the proper quality 
of heat, uniformly applied, and of the right degree? The elec- 
tric furnace for the heating of steel brings the answer. It over- 
comes most of the objections to the " combustion process " by 
introducing a new principle. 

Electric Heat. — The heat of the electric furnace is produced 
in an entirely different way from that of the process of combus- 
tion. Electric heat can be produced by means of the electric 
arc, as in the arc lamp, and by the resistance of a conductor, as 
in the incandescent lamp. It is the latter principle — due to 
its greater flexibility and convenience — that was utilized by 
Albert L. Marsh in the electric furnace developed by the Hoskins 



n6 



HEAT-TREATMENT OF STEEL 



Mfg. Co., Detroit, Mich., for the heat- treatment of steel. Fig. i 
shows a section of a furnace of one of the larger sizes, the cham- 
ber in this being 18 inches deep (front to back), 12 inches wide 
and 8 inches high. The relation of the various constructional 
parts is clearly shown; A shows the fireclay insulation; B, the 
carbon connector plates; C, the graphite bottom plates; D, 




A 



w 




,m. 



■J ff- m 

1 ; Lv,' ,7^ 



Machinery 



Fig. 1. Section through Electric Furnace 

the draft hole; £, the pyrometer hole; F, the electrodes; G, 
the resistor plates; H, fire sand; K, cement filling; L, the inlet 
for the water used for cooling the electrode clamps; M, the outlet 
for this water; N, the electrode clamps; and O, the pressure reg- 
ulating screws. The electrodes are surrounded by asbestos at P. 
The full length of the side walls and the entire roof of the 



ELECTRIC HEATING FURNACES 117 

chamber are formed by the heating elements ; the walls are com- 
posed of a series of thin carbon plates resting on the top of a 
heavy block of the same material, and the roof, of a thick graph- 
ite plate connecting these two columns at the top. One graphite 
electrode projects up to the middle of each side-wall plate and 
connects electrically, through water-cooled clamps at the lower 
end, with the source of energy. The chamber floor is of cement. 
Outside of the carbon plates there is a lining of the same material. 
This lining, with a carefully designed backing of heat-resisting 
material, retains the heat developed within the furnace. The 
counterweighted door fitted with a peep-hole serves as a quick 
access to the chamber, while in the rear wall are holes for the 
insertion of a pyrometer tube and for draft regulation. A rigid 
enclosing case of steel holds all parts securely. 

The principle of operation is simple. A heavy low- voltage 
electric current is supplied through the electrodes to the resistor 
plates forming the side walls of the working chamber. Heat is 
generated here, due to the resistance offered by these plates to 
the passage of the current. The electrical " resistivity " of the 
carbon causes each plate to heat exactly as the carbon filament 
in the incandescent lamp " lights " when the current is turned on. 
In addition to this action, advantage is taken, in the furnace, of 
a second form of electrical resistance — that of the contact of 
one plate with another. This may be readily varied by altering 
the mechanical pressure on the plate columns by means of the 
hand-screws. The turning of these, changes the resistance of 
the circuit and hence the resulting temperature produced. 

While the furnace is " electric " in its nature, it is not at all 
necessary that the hardening man handling it be an electrician. 
The simple electrical features of the furnace are quickly grasped. 
It is also safe, both because it practically eliminates the fire 
hazard and because it brings a corresponding protection to the 
operator. Normal working temperatures are acquired in a little 
over an hour's time after the switch has been closed. An aver- 
age of i2j-kilowatt energy consumption will maintain the cham- 
ber at approximately 2250 degrees F.; higher temperatures, up 
to 2500 degrees F., which is even above the requirements of high- 



Il8 HEAT-TREATMENT OF STEEL 

speed steels, or lower, as desired, may be obtained by increasing 
or decreasing the energy supply. 

The life of the various parts of the heating unit is shown to 
be from 150 to 200 operating hours for the side-wall resistor 
plates, and 500 hours for the electrodes. Based on ten-hour 
day operation, it is found that the upkeep cost of these items, 
even on this larger-sized furnace, is less than 40 cents a running 
day. The rest of the furnace does not depreciate rapidly. 

The Advantages of Electric Heating. — The atmosphere in 
the heating chamber of the electric furnace is inherently " reduc- 
ing " in its nature, due to the fact that the hot carbon plates 
absorb all of the atmospheric oxygen. By raising the door 
slightly, and opening the draft-hole at the rear, a slight current 
of air may be admitted which will counteract this tendency. 
Leaving the door open slightly more would allow an excess of 
air to enter, so that an oxidizing atmosphere could be produced. 
Between the extreme points fine shades of atmospheric conditions 
can be obtained. Thus the quality of the heat can be absolutely 
and easily regulated. 

Because of the arrangement possible with the electrical resis- 
tor, the heat may be generated within the working chamber itself. 
In the furnace described, the very walls of this chamber constitute 
the heat-generating device. The " resistivity " of the carbon 
plates is uniform — the same electric current runs through them 
all — with the result that an equal radiation of heat into the 
chamber takes place from practically every point in the walls. 

In any type of furnace the temperature is varied, within limits, 
by varying the amount of energy transformed into heat. The 
regulation of the energy supply thus becomes the means of the 
temperature control. The electric energy control lends itself 
with exceeding exactness to meeting this principle. In the 
furnace described both a very fine and a wide regulation of tem- 
perature may be obtained by slight variations in the mechanical 
pressure between the carbon plates. 

Commercial Importance. — Mr. Samuel S. Roberts, testing 
engineer of the Carnegie Steel Co., has been carefully investigat- 
ing the subject of steel heating furnaces. In an interesting report 



ELECTRIC HEATING FURNACES 119 

of a series of tests which he, together with a number of steel 
experts, recently made on the heat- treatment of carbon and 
high-speed steel tools in the electric furnace, he says in part: 

" A realization of the inadequacy of the prevailing furnace de- 
signs usually employed for the specific purpose of hardening and 
tempering specially formed tools of high-speed steel, such as 
formed milling and gear cutters, twist drills, taps, threading dies, 
reamers and other tools that do not permit of being ground to 
shape after being hardened, and where any melting or fusing of 
cutting edges must be prevented, has prompted the large tool 
steel consumers to welcome the advent of refined heating appli- 
ances, whereby the destructive influences hitherto encountered 
are eliminated. 

" The modern electric resistance furnace, with its perfect heat 
control, evenly distributed heat maintenance at any desired 
point, reducing atmosphere, absence of all products of combus- 
tion, and thermo-electric pyrometer for measuring the temper- 
ature, offers not only the most attractive method whereby the 
consumers of tool steel are insured maximum efficiency, but has 
caused the science of treating the rapid cutting tools to take a 
long step forward." 

Practical Application. — As to the cost of operation, it is a 
demonstrated fact that the higher the temperature it is desired 
to produce, the lower the cost of electric heat in comparison with 
fuel heat. At the lower ranges, considering only the production 
of the necessary heat alone and with electric power at the usual 
commercial rates, heat from this source costs considerably more 
than heat from fuels, especially the cheaper ones. Where water- 
power supplies of electric current are available, this ratio decreases 
in favor of electric heat; but the cost of producing the energy for 
the heat-treatment is only a part of that of the whole operation 
involved. When we consider that this broad factor includes the 
resulting service of the finished tool, as well as the labor, material 
and overhead charges to produce it, we see how comparatively 
small this part is. It is from such a view of the entire cost of 
production that the improved hardening of steel in electric heat 
is seen to be a real economy. The electric furnace, due to its 



120 HEAT-TREATMENT OF STEEL 

advantages, makes possible a higher quality of product than is 
possible with fuel heating. 

The use of electric furnaces for hardening and tempering steel 
has passed through the experimental stage and there are several 
types now on the market that have been made commercial suc- 
cesses. About 1 910, an electric furnace was placed on the market 
that uses a molten salt bath in which to heat the steel to any of 
the hardening temperatures used for carbon, alloy or high-speed 
steels. This electric salt-bath furnace is also used for any 
drawing temperature above the flash point of oil, while an oil 
bath type is used to heat steel to drawing temperatures below 
this flash point. In the salt-bath furnaces, the electric current 
is sent directly through the salt by placing electrodes inside of 
the bath on opposite sides, as explained in detail in a following 
chapter, while in the oil-bath furnaces the oil tank is heated from 
the outside. 

Since the introduction of the salt-bath furnace, the oven type 
of electric furnace has been perfected and installed by several 
manufacturers for commercial work. Small furnaces of this 
type have long been in use for laboratory and experimental 
work, but the principle used for heating, namely, sending the 
current through coiled resistance wires inside of the heating 
chamber, is too expensive for the larger furnaces used for com- 
mercial work, and the high temperatures required for high-speed 
steel cannot be reached without burning out the resistance wires. 
They are very successful, however, for small furnaces that are 
used at temperatures below 1800 degrees F. 

One of the largest installations of the modern oven-type of 
electric furnaces is the battery of electric hardening furnaces 
that are in daily use at the Timken-Detroit Axle Co.'s plant. 
These were manufactured and installed by the Hoskins Mfg. Co. 
of Detroit, Mich. Each separate unit consists of a furnace, 
switchboard and transformer, which stands back of the switch- 
board. On the board is placed the line switch for turning on 
and of! the current; a circuit breaker, adjusted to open the cir- 
cuit automatically in case of an excessive flow of current; an 
ammeter to show the amount of current being used; and a py- 



ELECTRIC HEATING FURNACES 121 

rometer to indicate the temperature inside of the furnace. No 
rheostat is required, as the furnace can be adjusted to regulate 
the amount of current that is necessary to produce any given 
degree of temperature. 

Heating Gears in the Electric Furnace. — The bulk of the 
work heated in these furnaces consists of the gears that enter 
into the construction of rear axle drives used on automobiles. 
It was found difficult to harden these large ring gears uniformly 
at accurate predetermined temperatures, without warpage, be- 
fore the electric furnaces were installed, but now most of the 
difficulties have been overcome and good results are obtained. 
The gears are first pre-heated to about 1200 degrees F. in an 
ordinary kind of furnace. Four gears are then placed on the 
thick iron disk in the electric furnace, and heated to the correct 
temperature for quenching to harden. The disk is slowly re- 
volved automatically, by mechanical means, to insure obtaining 
a uniform temperature around the entire circumference of the 
gears. To heat four carbon-steel gears, 11 j inches in diameter, 
from the 1200 degrees of the pre-heating furnace to the 1500 
degrees required to harden the gears, takes about twenty minutes. 
Prolonged heating does not harm the gears as the furnace is 
maintained at the correct hardening temperature and they will 
not be overheated, while the atmosphere is reducing and thus 
prevents the formation of any scale. If not left in long enough, 
however, they will not attain the temperature of the furnace and 
therefore the quenching will not give them the desired hardness. 

Action of Carbon in Heated Steel. — It is a well-known fact 
that steel absorbs various elements with which it comes in con- 
tact, when the conditions are favorable. As is well known, this 
is the principle of the carbonizing process. Many impurities 
which injure some of the good qualities possessed by the metal 
are absorbed under certain conditions, and several elements 
that improve some qualities are absorbed under other conditions. 
Some of these conditions are known and can be controlled, while 
many others are unknown or only partially known. 

Some beneficial elements, of which chromium is an example, 
can be injected into steel. Carbon flows through steel in a some- 



122 HEAT-TREATMENT OF STEEL 

what similar manner to the flow of electricity, although very 
much slower. If a piece of high-carbon steel and a piece of low- 
carbon steel are bound closely together and allowed to stand a 
long enough time, they will both have the same percentage of 
carbon. This would take many years at atmospheric temper- 
atures, but if the steel is heated, each degree of temperature 
increase accelerates the flow of the carbon. When the molten 
stage is reached, minutes will produce, in the equalization of the 
carbon content, what it might take centuries to produce at 
atmospheric temperatures. 

The principle here involved is that carbon flows to the body 
or element which has the greatest attraction for carbon. In 
carbonizing steel, a carbonaceous gas is produced, but the steel 
has a greater attraction for the carbon than the atmosphere in 
the carbonizing retort and hence it enters into a combination 
with the steel. Likewise, with two pieces of steel, the one low 
in carbon has a greater attraction than the piece high in carbon 
and draws this element away until the attractive force in each 
piece has been equalized. In manufacturing steel, the carbon 
is put into the molten metal and as the iron has a greater affinity 
for carbon than any of the elements in the slag, it combines with 
the iron and stays in the steel thus produced; hence, the correct 
percentage is found in the finished product. 

We often find decarbonized spots in steel that has been heated 
for hardening in ordinary fuel-heated furnaces. A condition 
has here been produced whereby some element has entered the 
heating chamber that has a greater attraction than iron for the 
carbon; hence enough carbon flowed out of the steel to satisfy 
this attraction for the time this condition prevailed. Time and 
temperature seem to be the two factors that control the rate of 
flow of the carbon. Salt and lead-bath furnaces may produce 
these decarbonized spots. Sometimes a pitting of the surface 
occurs, which means that some element is present that has a 
great attraction for iron and has eaten it away. 

In the electric furnaces previously referred to, no such elements 
seem to be present, as pitted and decarbonized surfaces are not 
found in the steels heat-treated in them. This applies to the 



ELECTRIC HEATING FURNACES 123 

carbon steels, which are heated to hardening temperatures as 
low as 1300 degrees F., and also to the high-speed steels that 
require a temperature as high as 2200 or 2300 degrees. Nearly 
all of the temperatures between these points are utilized, as the 
correct hardening temperature for some of the carbon steels is 
as high as 1500 degrees, while the alloy steels require from 1500 
to 1700 degrees and some high-speed steels need a temperature 
as low as 1750 degrees. The elimination of pitting and decar- 
bonizing is doubtless due to the fact that the oxygen in the air 
is not required to perfect combustion, and none of the products 
of combustion are present to attack the metal. As the steel 
leaves the furnace, no scale can be seen, because scale always 
comes from an excess of oxygen; neither is there any of the 
deposit that is sometimes produced by the products of com- 
bustion. 

Effect of Oxygen on Iron and Steel. — Under certain condi- 
tions oxygen has a great affinity for iron and penetrates steel 
much more rapidly in a moist atmosphere, or one that has been 
heated. If steel is left in a damp place, it is only a question of 
time when it will be reduced to an iron oxide in a powdered form, 
the degree of dampness being the factor that governs the speed 
of this reduction. Thus, if steel is immersed in water it de- 
composes much more rapidly than when left in the air. Salt 
water reduces the time of this decomposition, as the salt aids the 
oxygen in forming its combination with iron. Without the aid 
of moisture, any increase in the temperature of steel increases 
the ability of oxygen to unite with the iron. When the boiling 
point is reached, the metal is saturated with oxygen and other 
gases, but the bulk of these are expelled when the steel solidifies. 
Enough is often left, however, to considerably weaken its various 
physical properties. 

In furnaces that depend on combustion, or flames, for obtain- 
ing the necessary heat, a scale is liable to form on steel at the 
temperatures required for hardening, unless all of the oxygen is 
utilized to form the combustion and passes out of the vent as 
carbon dioxide (CO2). This scale injures the piece being hard- 
ened, as it reduces its size and makes the surface uneven. This 



124 HEAT-TREATMENT OF STEEL 

formation of scale proves that oxygen combines with steel at 
temperatures far below the melting point; hence the valves of 
such furnaces must be carefully adjusted so as not to deliver an 
excessive supply of air. 

In addition to the formation of scale, oxygen injures steel in 
other ways. When the steel is heated to the higher temperatures, 
oxygen may enter the open pores to form microscopic bubbles 
and thus reduce the cohesive force that binds the molecules of 
the mass together. It may take the form of occluded gas in 
combination with other gases, or it may form a ferrous oxide 
with the iron. In any of these forms, oxygen reduces the 
strength, wearing qualities and resistance to fatigue or torsion 
stresses. Steels containing oxides also rust more quickly than 
those that are practically free from them. It is not so much 
the oxygen itself that is injurious, as it is the oxides that it forms 
with other elements. 

The percentage of oxygen has heretofore been considered to 
be too small to be taken into account, but 0.05 per cent of oxy- 
gen is equal to 0.22 per cent of ferrous oxides, and this is sufficient 
to materially reduce the physical properties. As oxygen is a 
gas, it was difficult to analyze steels for this element, but with 
the new methods that have been devised, it has been found to 
be present in steel in larger quantities than was supposed, and 
efforts are now being made to reduce this impurity in all steels. 
That the pores of steel are opened by heat and allow the gases 
to enter or leave has been proved by a number of experiments 
where both cast and rolled steel have been heated in a vacuum. 
Under this condition, the gases began to leave the steel at tem- 
peratures between 300 and 600 degrees F. The volume reached 
a maximum at temperatures between 900 and 1000 degrees, and 
was then reduced to a minimum volume at about 1300 degrees. 
Another maximum point was reached at about 1450 degrees; 
then again reduced and again increased at higher temperatures. 
The greatest evolution of gas seemed to take place at the trans- 
formation point of the metal. That gases travel into the steel 
under atmospheric pressure is shown by the fact that oxygen 
combines with iron and raises blisters or scale and also that 



ELECTRIC HEATING FURNACES 125 

nitrogen penetrates steel that has been raised to temperatures 
around 2200 degrees. 

Effect of Nitrogen on Steel. — Nitrogen is just beginning to 
be recognized as an injurious element in steel. It now occupies 
practically the same position that sulphur and phosphorus did 
but a short time ago, when the chemist first proved that they 
were very injurious elements to the physical properties and 
pointed out that means should be devised to keep them as low 
as possible. Then the practical steel makers scoffed at the 
chemists for saying that such small percentages of any element 
could weaken the steel and if it did their ancestors would have 
made it known. One does not have to be very old to remem- 
ber the time when such talk was prevalent in the steel mills. It 
has been proved, however, that nitrogen is as liable to cause 
brittleness and " cold shortness " as is phosphorus, and is as 
injurious. 

The better grades of steel contain from 0.005 to 0.025 per cent 
of nitrogen, while the cheaper grades contain between 0.010 and 
0.065 P er cent. Each increase in the percentage causes the 
elongation to diminish rapidly and the ductility to be reduced. 
At first, only a slight decrease occurs in the toughness, but the 
decrease becomes more rapid as the percentage of nitrogen in- 
creases. When the carbon content of steel is high, 0.035 P er 
cent of nitrogen will cause the elongation and contraction to be- 
come practically nil, while in medium carbon steel it may take 
a nitrogen content of 0.050 per cent to accomplish this, and 0.065 
per cent in low-carbon steels. Steels made in the resistance 
type of electric furnace contain only traces of nitrogen, but 
those made in the presence of basic slag in the arc type may 
contain an injurious amount. 

As four-fifths of the air is composed of nitrogen, by volume, 
consuming the oxygen with a flame would leave much of the 
nitrogen free to enter the pores of steel that is heated to the hard- 
ening temperatures in ordinary oven furnaces, especially when 
the steel is heated to the 2200 degrees F. or more required for 
hardening some brands of high-speed steel. Decarbonization is 
doubtless due to the nitrogen that may be occluded in steel 



126 HEAT-TREATMENT OF STEEL 

combining with the carbon to form methane, which escapes when 
the metal is heated. As the resistance type of electric furnaces 
does not consume any of the oxygen in the air, none of the nitro- 
gen is set free. Thus a condition that allows the nitrogen to 
penetrate the steel is not created, even though the steel be 
heated and the pores open. As steel pieces do not warp as much 
in this type of electric furnaces, as in coal-, oil- or gas-fired 
furnaces, it may be because the metal is more dense, owing to 
its not absorbing any gases. 

The combustion type of furnace, especially when using coal 
as fuel, gives off many elements as products of combustion 
which may be injurious to steel. Among these may be men- 
tioned such hydrocarbons as anthracine, naphthalene, toluene, 
benzine, methane, ethane, etc., while acetylene, benzole and 
sulphur are other products sometimes present. With a large 
amount of hydrocarbons present in a furnace, it is almost im- 
possible to prevent carbonization in the steels, as the gases that 
are commonly present, other than nitrogen, have no other effect 
on steels. Thus, some parts of the piece being hardened would 
be carbonized more than others and cause a variation in the 
hardness. To prevent this, the mufiie type of furnace should 
be used. The instruction given with all high-grade steels is to 
heat the steel in a mufiie furnace so that the products of combus- 
tion, or the flame, cannot attack the metal. The construction 
of the electric furnace, however, is such that it forms its own 
mufiie and the expense of renewing muffles is done away with. 

Current Consumption and Operating Cost. — A resistance 
furnace of the type described, with a heating chamber 1 2 inches 
wide, 18 inches deep and 8 inches high, can be brought up to 
the higher temperatures required for high-speed steel, with 30 
kilowatts, in one hour and fifteen minutes, and then be main- 
tained at this temperature with about one-half of this current. 
With an average amount of steel to heat, it can be operated 
during a ten-hour day with about 145 kilowatts. The heat insu- 
lation is so effective that furnaces have been closed after turning 
off the current, and at the end of twelve hours the temperature 
had only dropped to between 700 and 800 degrees. Smaller 



ELECTRIC HEATING FURNACES 127 

furnaces with a heating chamber 7 inches wide, 12 inches deep 
and 5 inches high, only use one-half the current mentioned. 

The current cost for an electric furnace is undoubtedly higher 
than the fuel cost for a gas-, oil- or coal-fired furnace of the same 
size, but when all other things have been taken into consider- 
ation, this current cost is minimized and the electric furnace 
can be made to compete commercially with other kinds. This 
is especially true when quality is a factor in the work heat-treated. 
The ease with which the temperature can be controlled and ac- 
curately maintained; the absence of scale or pitting; the decrease 
in warpage; the cleanliness of the work due to the absence of 
deposits of any kind; the overcoming of decarbonized spots; 
the lowering of the penetration of gases or other injurious ele- 
ments, such as sulphur; and the uniformity in the hardness or 
temper of the steel, are all things which should be figured against 
the cost of fuel. In addition to this, the electric furnace can be 
so heat-insulated that the hand can be held on the outer shell 
or case without burning. This lowers the temperature of the 
hardening room and makes the working conditions so much 
better that operators are enabled to turn out more work. One 
company cites a case where three tons of cold-rolled steel were 
heat-treated per week, in the shape of razor blades, in six elec- 
trically-heated furnaces with two operators; whereas, before 
the installation of electric furnaces, it required fifteen men, and 
sixty-five furnaces that used gas and blast, for the hardening 
operations on the same tonnage. While this might be a case 
where the work was particularly adapted to electric furnaces, 
many others can doubtless be found, where a saving could be 
effected when all of the factors are figured into the cost. 

A report from the Timken-Detroit Axle Co. relating to the 
results obtained with the furnaces in their plant states that 
with the four furnaces mentioned each hour of the ten-hour work 
day averages 75 pounds of steel gears heated to the hardening 
temperature in each furnace. The average current consump- 
tion is 13 K. W. per hour for each furnace, costing ij cent per 
K. W. The average cost per pound of steel hardened is a trifle 
more than 0.2 cent. 



CHAPTER VII 

THE METALLIC SALT-BATH ELECTRIC HARDENING 
FURNACE 

In externally-fired furnaces, the heat losses are always con- 
siderable, and only a small part of the energy used in heating is 
utilized for raising the temperature of the metal to be hardened. 
There is also a disadvantage in employing gas- or oil-fired furnaces 
in that the high temperatures rapidly destroy the crucibles. 
Electric hardening furnaces, therefore, possess marked advantages 
for this work over the various types of externally-fired furnaces. 
The electric furnace described in the following has been brought 
out by the Allgemeine Elektricitats-Gesellschaft of Berlin, Ger- 
many, and in this country by the General Electric Co., Schenec- 
tady, N. Y. Briefly described, the furnace consists of a bath of 
melted metallic salts contained within a firebrick crucible, in- 
side of which, at two opposite sides, are fixed electrodes of iron 
very low in carbon, the melting point of which is higher than 
that of ordinary steel. This crucible is surrounded by a thick 
layer of asbestos, which is, in turn, imbedded in a layer of some 
heat-insulating material, the whole being held together by a 
steel case. The walls of the furnace are made so thick in relation 
to the dimensions of the crucible that the steel case of the appa- 
ratus may be touched with the hand without injury after having 
been in operation for hours, at the highest temperatures required 
in the hardening room. 

The soft iron supply conductors to the electrodes are connected 
to the secondary copper bars of a regulating transformer which 
transforms the normal voltage to the low voltage (5 to 70 volts) 
employed in the operation of the furnace. A typical arrange- 
ment of the equipment of a large works has the furnaces pro- 
vided with a hood in a central position, and a quenching tank 
immediately beside the furnace on one side. By this latter 

128 



ELECTRIC HEATING FURNACES 1 29 

arrangement the change in temperature caused by carrying 
pieces from the furnace to the water tank is reduced to a mini- 
mum. The tank is supplied with heating and cooling coils with 
steam or cold water, so that the temperature of the quenching 
bath can be easily regulated. 

Requirements of Hardening Furnace. — A great many factors 
must be considered in the development of the design of an elec- 
trical hardening furnace. The practical requirements which 
should be fulfilled by an ideal hardening furnace may be sum- 
marized in a general way as follows: 

1. The furnace should make it possible to obtain all hardening 
temperatures required in industrial practice, thus having a range 
of from 1400 to 2450 degrees F. 

2. The steel should be heated to the required temperature 
easily and rapidly. 

3. The temperature of the steel should be easily ascertained, 
and it should be possible to keep it well under control within a 
margin of, say, 50 degrees F. above or below the exact temper- 
ature required. 

4. The steel must be equally heated all over, notwithstanding 
different cross-sections of the object, thus preventing the over- 
heating and burning of edges and points. 

5. During the heating process foreign matter must not come 
in contact with the steel so as to change its carbon content, or 
affect it in other respects. 

6. It should be possible to place the cooling tank close to the 
furnace in order to minimize the loss of heat during the transfer, 
and avoid the oxidizing influence of the air. 

7. The furnace should not give off obnoxious or poisonous 
vapors of lead, potassium-cyanide, etc. 

8. The total operating cost incident to the hardening process 
should be low. 

The electric hardening furnace described in this chapter is 
claimed to fulfill to a considerable extent all of the previous 
requirements, and a general review of the advantages of electric 
hardening will be given. It should be understood, however, 
that many of the claims made at the introduction of this furnace 



130 



HEAT-TREATMENT OF STEEL 



have not been substantiated by experience, as will be explained 
in the latter part of this chapter. 

Description of Hardening Furnace. — In Fig. i are shown 
vertical and horizontal sections of the hardening furnace. A 
bath of metal salts is contained in a fireclay crucible. Current 
is transmitted to the bath by two electrodes made of Swedish 
ingot iron, which is characterized by a particularly low per- 
centage of carbon, and therefore has a melting point of as high 
as 2700 to 2900 degrees F. As shown in the horizontal cross- 
section, the electrodes end in iron terminals sweated in turn to 
copper conductors. The crucible is surrounded by an asbestos 








SECTION X-X 



Machinery 



Fig. 1. 



Horizontal and Vertical Sections through a Metallic Salt-bath 
Electric Hardening Furnace 



lining, a fireclay receiver, and a layer of insulating material, 
the whole being contained in a cast-iron case. This construction 
greatly reduces the radiation losses, and after ten hours' operation 
of the furnace at about 2450 degrees F., the cast-iron case has a 
temperature of only from 85 to 105 degrees F. Over the bath a 
sheet-iron hood is placed fitted with chimney and damper. These 
furnaces are made in several different sizes. In the smallest size 
the inside dimensions of the crucible are about 5 inches square 
by 5 inches deep, and in the largest size 12 inches square by 15 
inches deep. The approximate consumption of current in kilo- 
watts at various temperatures for the large and small furnaces 
is given in Table I. 



ELECTRIC HEATING FURNACES 131 

The best composition of the bath depends mainly on the 
temperature required for the hardening. Table II gives the 
composition of various salts to be used for different processes. 
The conductivity of the salts at normal temperature is very 
small, while at high temperatures (when in a melted condition) 
they offer to the electric current a comparatively low resistance. 
When the mixture is sufficiently hot, the bath, therefore, forms 
an electric conductor, and each part of the bath produces its own 
heat. This feature distinguishes this class of electric furnaces 
from other types. 

The heating of the salts prior to their becoming highly con- 
ductive is done by means of an auxiliary electrode and a piece 
of arc lamp carbon. The carbon is first pressed against one of 
the main electrodes and soon reaches a white glow, melting the 
salts immediately about it. The auxiliary electrode, which con- 
sists of an iron stick fitted into a wooden handle, is then drawn 
towards the other main electrode, the molten salt trailing behind 
it until a bridge is established between the two main electrodes. 
The current which now passes through the molten salt continues 
to raise the temperature of the bath until the required heat is 
attained. The articles to be heated are dipped into the bath, 
suspended by thin iron wires or held by tongs, and are allowed 
to remain in the bath until uniformly heated throughout. 

The most striking feature of this furnace is the possibility of 
securing uniformity in temperature throughout the whole bath. 
Careful measurements with a pyrometer of the thermo-couple 
type at various parts of the bath have shown that the tempera- 
ture varies only 5 or 6 degrees F., except in an upper layer about 
I inch thick, where, owing to radiation, the temperature is from 
20 to 35 degrees F. lower. Alternating and not continuous 
current should be used; all frequencies between 25 and 60 cycles 
may be applied; with less than 25 cycles electrolytic phenomena 
appear. The furnace, having only two electrodes, is suitable for 
single-phase currents only. If a single-phase supply is not avail- 
able, a converter must be installed. 

An important part of the hardening installation for electric 
furnaces is the pyrometer. The most reliable results for the tern- 



132 



HEAT-TREATMENT OF STEEL 



peratures in question are obtained by instruments of the thermo- 
couple type, but the instruments must be such that the terminals 
are kept outside of the destructive influence of the heat. The 
thermo-couple used is platinum — platinum-rhodium, protected 
by Marquardt composition and steel. A steel cylinder protects 
the parts projecting from the bath. This cylinder gets white 
Table I. Current Consumption of Electric Furnaces in Kilowatts 



Temperature of 

Bath, 

Degrees F. 


Size of Crucible of Furnace, Inches 


5X5X5 


6X6X7 


8X8X11 


12 X 12 X 15 


1400 
I550 
2IOO 
23SO 


2-5 
3-o 

5-5 
7-5 


3-5 

4-5 

9.0 

12.0 


7-5 

8-5 

16.0 

22.0 


17-5 
20.0 
36.0 
48.0 



hot and deteriorates unless protected against the oxidizing in- 
fluence of the air. 

When the salts are melted, the voltage necessary for maintain- 
ing the temperature is from 5 to 30 volts, while the heating-up 
voltage is about 70 volts. Such low voltages are not available 
from ordinary supply systems, and consequently a transformer 
Table II. Temperature and Composition of Hardening Bath 



Process 


Temperature, 
Degrees F. 


Salts 


Tempering steel • 

Annealing copper, al- 
loys, etc 


400 to 1075 

1200 to 1650 

1400 to 2000 
1900 to 2450 

2700 to 2900 


Sodium nitrate and potas- 
sium nitrate 

Sodium chloride or sodium 
chloride and potassium 
chloride 

Potassium chloride and ba- 
rium chloride 

Barium chloride 

Calcium fluoride or magne- 
sium fluoride 


Hardening regular car- 
bon steel 


Hardening high-speed r 
steels C 



must be used. The heat developed, and consequently the tem- 
perature of the bath, depends on the voltage. If it is desired 
to alter the temperature, this can therefore be done by a varia- 
tion of the voltage. The use of the transformer makes the 
voltage control comparatively simple. 



ELECTRIC HEATING FURNACES 1 33 

The Hardening Process. — When heating carbon steel for 
hardening, it is advisable to heat it as rapidly as possible, because 
the prolonged influence of the heat seems to affect the chemical 
constitution and mechanical structure. In most cases it is ad- 
visable to pre-heat the steel to a certain temperature before the 
final heating in the bath takes place. This pre-heating should 
be done thoroughly in order to make sure that all portions are 
well heated. Unless this is done during the pre-heating period 
the heat during the main heating period must be directed to the 
parts not properly heated, which lengthens the process and may 
cause damage to the external portions of the tool, such as edges, 
projections, etc. 

Each brand of steel requires a certain temperature to which 
it should be heated for hardening. For high-speed steels this 
temperature is from about 1800 to 2375 degrees F., and for 
carbon steels from about 1300 to 1650 degrees F. Generally 
speaking, the cooling process for alloy steels need not be so abrupt 
as for carbon steels. Instead of quenching the hot tool in water 
or oil, it is sufficient to expose it to a current of air or to dip it 
into molten tallow. 

Advantages of Electric Furnaces. — A great advantage of the 
electric furnace is that it is possible to cover a wide range of tem- 
peratures with one equipment by only changing the composition 
of the bath. The tool can thus stay in the bath until it has 
acquired the temperature of the bath, and does not need to be 
removed before it has assumed the temperature of its surround- 
ings, which is quite commonly the case in other heating processes. 
With the electric hardening bath, having a predetermined tem- 
perature, less dependence is placed upon the skill of the operator, 
and no account need be taken of the fact that smaller cross- 
sections heat up quicker than larger ones. 

When dipping cold steel into the heating chamber, the temper- 
ature of the latter must drop. In fact, with gas-fired furnaces 
and salt baths it falls rapidly, unless the uncertain procedure of 
increasing the gas supply is resorted to. In the electric furnace, 
when the tool is dipped into the salt, the level of the salt bath 
rises, and the current of heat produced increases automatically. 



134 HEAT-TREATMENT OF STEEL 

Besides, when it is necessary to immerse large solid masses, the 
current supply can be easily increased by the regulator, thus 
preventing a drop in temperature. 

Of course, the smaller cross-sections of the tool will heat up 
quicker than the larger ones in the electric furnace as well as 
in other heating furnaces, but the delicate parts will not over- 
heat because they cannot assume a higher temperature than that 
of the bath itself. The bath equalizes all differences in temper- 
ature, and in a very short time heats the whole mass uniformly. 
This explains the very small loss from overheating in electric 
furnace plants as compared with others. 

While the tool is in the bath, the air is, of course, prevented 
from coming in contact with it, but a thin coating of salt pro- 
tects it still further when on its way from the bath to the cooling 
tank, and falls away first when the object is placed in the cooling 
liquid. This is a great advantage over all types of open-fire 
or muffle furnaces, but is common to all bath-type furnaces. 
Metal salts, moreover, offer the advantage that they do not 
give off poisonous gases, and unlike lead, they can be obtained 
comparatively pure at a reasonable cost. The salt coating also 
breaks up entirely in the cooling liquid, while when tools are 
heated in lead, small particles of it sometimes stick to the steel, 
leaving soft spots on the hardened surface. 

During the heating-up period or when a certain temperature 
is exceeded, the melted salts give off a small amount of vapor, 
and therefore a hood and chimney are provided for the furnace, 
but during normal operation there are scarcely any vapors pro- 
duced. The hood offers the further advantage that the radiation 
from the bath surface can be used for the pre-heating of the 
articles to be hardened. A grate may be fixed in the hod in 
which the articles are placed, prior to being dipped in the bath. 

Comparative Operating Cost. — The parts subject to wear in 
an electric furnace are the crucible and electrodes. The crucible 
has been found to have a life of from 1200 to 1800 hours at a 
temperature of 2350 degrees F., and up to 3000 hours at lower 
temperatures. This is much longer than with muffle furnaces, 
which is probably due to the absence of the destructive influence 



ELECTRIC HEATING FURNACES 135 

of the gases of combustion, and to the fact that the crucible does 
not transmit the heat from the outside to the inside. The most 
sensitive part of the electrodes is that which projects over the 
level of the bath, and which is protected by exchangeable tips. 
These tips have a life of from 400 to 800 hours, and the cost of 
their replacement is as low as fireclay for other furnaces. The 
amount of salts lost by evaporation and waste under ordinary 
working conditions in a furnace 8X8X11 inches amounts to a 
little more than one pound for ten hours' continuous operation. 

The ease with which the electric furnace can be handled makes 
it possible to use cheaper labor than that employed in plants 
where the success of the work depends on the skill of the opera- 
tor. The speed of the hardening process is also much greater, 
and therefore a larger number of pieces can be handled per hour. 

Even at a temperature of 2400 degrees F., attainable in labor- 
atory tests, but not usually employed in commercial hardening, 
the damage to the crucibles of the electric furnace is very small. 
Working ten hours a day with this temperature, a crucible will 
last six months, and for ordinary hardening temperatures, fifteen 
months. 

Results obtained. — By means of this process, it has been 
possible to harden large high-speed steel milling cutters in about 
half an hour, including the time for pre-heating, which takes 
the greatest part of the time. Bringing the cutters up to a tem- 
perature of 750 degrees F. constitutes this pre-heating. After 
that, it takes only about a minute to bring an average-sized 
cutter to 1400 or 1500 degrees F., and then another minute to 
bring it up to about 2370 degrees F., which is considered the right 
hardening temperature for some brands of high-speed steel. At 
this temperature, however, the barium chloride attacks the steel, 
as will be referred to in detail on the following pages. The time 
stated above refers to average-sized and heavy milling cutters, 
whereas it only takes from 6 to 10 minutes to bring a small mill- 
ing cutter to the right temperature in the electrically heated 
salt bath. 

In regard to cooling the cutters, it has been found that when 
high-speed steel tools are cooled in an air blast, any moisture 



136 HEAT-TREATMENT OF STEEL 

coming in contact with the hot tool has a tendency to crack it, 
so that it becomes necessary to dry the air before it enters into 
the nozzles. It has also been found that it is absolutely impos- 
sible to cool a cutter which has a very heavy body and fine teeth 
in the air blast, as the heat from the central portion is not ex- 
tracted fast enough, and therefore does not permit a sufficiently 
rapid cooling of the teeth to insure proper hardening. For this 
reason, some firms have adopted a method of cooling the cutters 
from the hardening heat of 2370 degrees F. to a temperature of 
about 1 100 degrees F. by quenching in an electrically heated salt 
bath. After having been cooled to about 1100 degrees F. in the 
bath, the cutters are allowed to cool down slowly in the air, and 
the whole process has the advantage of being cheap and reliable 
as well as effecting a considerable saving in time. 

It must, however, be understood that electrically heated 
barium salt baths are advantageous to use only when a large 
quantity of tools is to be hardened, because this method will 
otherwise prove expensive. It has also been remarked that the 
electrically heated bath is more advantageous for heavy than for 
small tools, but it is not clear why the process should be thus 
limited to the former class of tools. The disadvantages of the 
barium-chloride bath will now be taken up. 

The Use of Barium Chloride for Heating Steel for Harden- 
ing. — As is well known, high-speed steel requires to be heated to 
a much higher temperature for hardening than does ordinary 
carbon steel. While a heat of from 1400 to 1600 degrees F. is 
sufficient for tools made from carbon steel, a heat of from, at 
least, 1800 to 2200 degrees F. is required in order to satisfactorily 
harden high-speed steel tools. The ordinary lead bath com- 
monly used for heating carbon steel tools cannot be used at such 
high temperatures as these, and as it is, in general, unsatis- 
factory to heat the tools in an oven furnace, owing to the diffi- 
culty of correctly determining the hardening temperature when 
the tools are heated in this way, some heating medium has been 
sought which could stand high temperatures and in which the 
pieces to be hardened could be immersed so as to obtain a uni- 
form heat without danger of burning delicate points or cutting 



ELECTRIC HEATING FURNACES 137 

edges — a danger which is always present when high-speed steel 
tools are heated to a high temperature in an open heating furnace. 
It has been believed that a satisfactory heating medium had 
been found in barium chloride, and this medium has been, and 
is still, used to a considerable extent both in this country and 
abroad; but the results obtained have not been as favorable as 
was at first expected, and many users of barium chloride have 
abandoned its use on account of the difficulties met with. 

It appears that tools heated for hardening in a crucible con- 
taining barium chloride have a soft scale or film of soft metal, 
probably 0.003 to 0.006 inch deep, all over the surface of the 
tool. Careful experiments have been made to ascertain as 
nearly as possible the conditions which contribute to produce 
such unsatisfactory results. Comparison has been made between 
tools made from the same material of which some were hardened 
by heating them in barium chloride and some in an oven furnace. 
The results of these experiments are recorded in the following. 

In order to make the tests as simple, and at the same time as 
conclusive, as possible, pieces of high-speed steel, | inch thick, 
were cut off from one bar of steel. These pieces were hardened 
by heating some of them in a common oven furnace, and others 
in barium chloride melted in a graphite crucible placed in a gas 
furnace. The pieces were heated directly from the room tem- 
perature to the hardening temperature, no pre-heating being 
resorted to. The barium chloride used was chemically pure. 
The temperatures were recorded by a Bristol pyrometer, and the 
hardness tests were made on a Shore scleroscope. After heating, 
the pieces were immersed in a cooling bath consisting of cotton- 
seed oil at a temperature of 100 degrees F. The temper was then 
drawn in an oil tempering bath at 500 degrees F., a temperature 
which is not too high for the higher grades of high-speed steel, 
although it would be excessive for ordinary carbon steel. 

When the pieces were heated in the oven furnace, the operator, 
an experienced hardener of this kind of steel, used his own judg- 
ment as to when to remove the piece from the furnace and plunge 
it into the hardening bath, but the time required for the piece 
to acquire proper hardening heat was recorded, and is given in 



*38 



HEAT-TREATMENT OF STEEL 



the accompanying table. The degree of heat as given, is the 
heat of the furnace as recorded by the pyrometer, but it is evi- 
dent that in the case of a piece of steel heated in an oven furnace 
and removed according to the judgment of the operator, there 
may be a slight variation between the heat of the furnace and the 
heat of the piece itself. When the tools are heated in the barium 
chloride bath, the temperature of the piece and the bath will, 
of course, be the same, provided the piece is permitted to remain 



Table Showing the 


Hardness of High-s 


peed Steel Heated 


for Hardening 






Under Different Conditions 






Heated in Oven Furnace 


Heated in Barium Chloride 






Left in Bath 10 


Left in Bath 18 








Minutes 


Minutes 


Heat of 

Hardening 

Bath, 


Time Test 
Piece was 


Degree of Hardness on 
Scleroscope Scale 






Degree of Hardness 


Degree of Hard- 


Degrees F. 


Left in 
Furnace, 




on Scleroscope Scale 


ness on Scler- 
oscope Scale 




Minutes 


















Face of 


Back of 


Face of 


Back of 


Face of 
Test 
Piece 


Back of 
Test 
Piece 






Test Piece 


Test Piece 


Test Piece 


Test Piece 


1700* 


9H 


Soft 


Soft 


Soft 


Soft 


Soft 


Soft 


1800 


7 


83.5 


85 


92 


91 


93 


90 


1900 


6 


91 


86 


93 


91 


9i 


92 


2000 


6 


90 


86 


91 


87 


92 


89 


2IOO 


5^2 


90 


9i 


91 


82 


87 


74 


2200 


5 


93 


9i 


88 


82 


86 


70 


2300 


4%t 


93 


93 


73 


64 


74 


62 


2400 


3H 


92 


93 


65 


65 


63 


57 



* At this temperature the steel would not harden, and, therefore, no scleroscope tests were 
made. 

t This sample was burnt and pitted, indicating that it had been kept in the fire too long. 

in the bath long enough, which was the case in the experiments 
described. 

After the pieces had been hardened and tempered as de- 
scribed, an amount equal to 0.005 mcn was ground off from one 
side of the pieces, which we will call the face, and an amount of 
0.002 inch was ground off from the other side, the back. The 
surfaces presented to the scleroscope were thus perfectly smooth 
and uniform, but it should be noted that less of the soft scale, 
mentioned in the foregoing, was removed from the back of the 
pieces than from the face. The pieces were now subjected to 



ELECTRIC HEATING FURNACES 139 

scleroscopic tests, carefully recorded and repeated several times. 
The results of these tests are given in the accompanying table, 
the values given being the average of the several readings. 

It should also be mentioned that the pieces heated in the ba- 
rium chloride at 2100 to 2400 degrees F. were found to be pitted, 
and small beads of a metallic structure adhered to the pieces. 
Similar small pieces were found in the bottom of the crucible 
after all the test pieces had been hardened. This residue was 
chemically analyzed and was found to consist principally of 
ferro-tungsten, the analysis showing tungsten, iron and carbon 
to be present. The carbon content was about 3.3 per cent, 
tungsten 9.8 per cent, and iron 86.9 per cent. 

Several interesting and instructive conclusions with relation 
to the heating of high-speed steel in an oven furnace, and the 
action of barium chloride as a heating medium for high-speed 
steel when hardening, may be drawn from the results recorded 
in the table. It will be seen that when heating in an oven 
furnace, the results obtained were almost uniformly better 
according to the heat at which the pieces were hardened. The 
higher the heat, the higher the scleroscopic test number. This 
result is in thorough harmony with the general principle that the 
higher the heat at which high-speed steel tools are hardened, 
the better their cutting and " standing up " qualities. When 
the pieces were heated in barium chloride, however, a result 
entirely different was obtained, and at temperatures of 2100 to 
2400 degrees F., the results were, in general, very unsatisfactory. 
In the case where the pieces were permitted to remain 18 min- 
utes in the heating bath it will be seen that the face of the piece 
is almost uniformly softer, the higher the hardening heat. This 
may be taken to indicate that there still was some of the soft 
scale left, even after having removed an amount equal to 0.005 
inch by grinding. A file test on the surface, however, could 
not detect this scale, as the surface seemed glass-hard. 

The feature which will particularly be noticed in studying the 
table is that in almost every case the back, where only an amount 
of 0.002 inch was removed, is softer than the face of the test 
piece. It is evident that this is due to the fact that the soft 



140 HEAT-TREATMENT OF STEEL 

scale is deeper than 0.002 inch, and has not been entirely removed 
by the grinding on the back; whereas the face, where an amount 
of 0.005 inch has been ground off, is practically freed from the 
soft scale, and hence shows a greater hardness when tested by 
the scleroscope. The influence of this soft film is especially 
apparent when the steel is hardened at a temperature of 2100 to 
2400 degrees F. 

Having ascertained through the tests mentioned that barium 
chloride had a detrimental influence upon the hardness of high- 
speed steel heated in it at high heats (2100 to 2400 degrees F.), 
tests were next made to ascertain the influence on the cutting 
qualities of tools hardened either by heating in barium chloride 
or in an oven furnace. These tests proved conclusively that the 
tools heated in the barium chloride bath did not stand as high a 
cutting speed as did those hardened by heating in an oven fur- 
nace. The ferro-tungsten found in the bottom of the crucible 
indicates that, particularly at high heats, some of the tungsten 
and carbon is removed from the tools into the bath, thus chang- 
ing the structure of the surface of the tool being heated. When 
an amount of, say, 0.010 inch is ground off from the cutting edges 
of tools, the influence of the heating in barium chloride is less 
noticeable — in fact, sometimes not noticeable at all — but 
when the tools cannot be ground after hardening, barium chlo- 
ride is not a heating medium which can be recommended under 
any circumstances. The change of the structure on the surface 
of the tool explains why tools heated in barium chloride cannot 
stand up at as high speeds as those heated in an open fire. 

Another disadvantage met with in the use of barium chloride 
is that the residue of ferro-tungsten found in the bottom of the 
crucible seems to have a deteriorating influence on the crucible, 
" eating " through it in a comparatively short space of time. 
As a general conclusion it may be stated that whenever barium 
chloride is used as a heating bath, it should never be permitted 
to reach a temperature of more than 2050 degrees F. 

Barium Chloride in the Electric Hardening Furnace. — The 
difficulties met with in the use of barium chloride in a crucible 
heated in a gas furnace are still further accentuated when using 



ELECTRIC HEATING FURNACES 14 1 

an electric hardening furnace of the type employing a barium 
chloride bath as the heating medium. 

When steel is heated in barium chloride in an electric furnace, 
the current apparently passes directly through the steel and the 
pieces are heated not only by the heat imparted to them from 
the barium chloride bath, but also by the resistance to the elec- 
tric current passing through the steel itself. That this must be 
the case is indicated by the fact that tools heated in an electric 
furnace are brought up to the proper temperature for hardening 
in approximately one-third of the time which is required for 
heating them in a barium chloride bath of the same temperature 
contained in a graphite crucible heated in a gas furnace. As 
an example it may be mentioned that certain tools which must 
remain sixteen minutes in a barium chloride bath in a gas furnace 
can be heated in the bath in the electric furnace in from four to 
five minutes. The barium chloride bath in an electric furnace 
does not cool down to the same extent when the pieces to be 
hardened are immersed, as does the heating bath in the gas 
furnace. This also indicates that in the electric furnace the heat 
required for bringing the steel to a hardening temperature is 
only partly derived directly from the bath in the electric furnace ; 
while all of the heat required must be given out by the barium 
chloride bath in the crucible in the gas furnace. These facts 
make it conclusive that there is an entirely different action in 
the heating of steel immersed in a bath of the same character 
in an electric furnace than there is when it is immersed in a bath 
contained in a graphite crucible. The rapidity with which tools 
to be hardened can be heated in an electric furnace has been 
quoted as one of its principal advantages, and so it would be 
were it not for the fact that the surface of the steel deteriorates 
under the action of the bath, the bath in turn deteriorating under 
the action of the electric current. 

On tools which are ground after hardening, there does not 
seem to be any difference between the hardness of those which 
have been hardened in barium chloride in a crucible and those 
heated in the same medium in the electric furnace. The tools 
can be run at the same cutting speed and will stand up equally 



142 HEAT-TREATMENT OF STEEL 

well; but when the tools cannot be ground on the cutting edges, 
as in the case of formed milling cutters, knurls, taps, dies, etc., 
then the tools heated in the electric furnace are decidedly inferior, 
especially under certain conditions which will be more thor- 
oughly explained in the following, there being a thin scale or 
film on the outside which is entirely too soft to possess proper 
cutting qualities. 

Experiments have been undertaken in order to determine, to 
some extent at least, the causes of the difficulties met with in 
heating tools in the electric hardening furnace. No conclusive 
answers can, perhaps, be given to all of the questions which may 
be asked in this connection, but the results of the experiments 
give at least a clue to the cause of the trouble, and further exper- 
iments might be made which would give still more conclusive 
results; if methods can be developed which will remedy the 
defects and make it possible to heat steel in a barium chloride 
bath in an electric furnace without having to contend with the 
soft scale on the surface of the steel, the electric furnace would 
present the best means for heating tools in a bath of high tem- 
perature, on account of the decided difference in the time required 
to bring the tools to the proper hardening temperature. 

It has been found that barium chloride when used in a graph- 
ite crucible in a gas furnace slightly deteriorates when it has 
been used for a number of days ; but the difference in the results 
obtained when heating in a bath of entirely new barium chloride 
and a bath which has been in use for several days, say from six 
to ten days, is so small that ordinarily no attention need be paid 
to it. Some users of the barium chloride bath, however, have 
been in the habit of changing the bath every day, using new 
barium chloride at all times. This practice is, of course, very 
expensive, and the advantages gained are too small to warrant 
the added cost. When barium chloride is used in an electric 
furnace, however, it deteriorates very rapidly, so that it is prac- 
tically useless for its purpose after a couple of days' use, and after 
having been used for a week it may be stated without exagger- 
ation that it is entirely unsuited for any further use. Further- 
more, the barium chloride which has been used in a graphite 



ELECTRIC HEATING FURNACES 1 43 

crucible generally has an almost white color after several days of 
use, whereas that used in the electric furnace is of a dark gray 
color. This difference in color is apparently due to the fact that 
the barium chloride in the electric furnace dissolves the ferro- 
tungsten which, as previously mentioned, is found in the bottom 
of the crucible when heating steel in a gas furnace; or possibly 
the soft iron electrodes are partly dissolved by the barium chlo- 
ride. The absence of a precipitate in the electric hardening 
furnace and the gray color of the bath would seem to make it 
safe to draw this conclusion. 

As regards the deterioration of the barium chloride after a 
few days' use in the electric furnace, several interesting facts 
have been noted. When steel is hardened after having been 
heated in a bath consisting of new barium chloride which has 
been used but one or two days, it seems to acquire a satisfactory 
hardness, at least as satisfactory as when heated in the same kind 
of a bath in a crucible heated in a gas furnace. There is, of 
course, a very thin soft scale, the same as on all tools heated in a 
barium chloride bath, but this scale is so thin that it cannot be 
detected with a file test, and hence can be considered of no con- 
sequence. After the first day or two, the influence of the barium 
chloride in conjunction with the electric current on the surface 
of the steel is considerably augmented; and when the barium 
chloride has been in use in an electric furnace for about a week, 
a scale from 0.005 to 0.010 inch deep can be easily detected. A 
scale of this thickness, of course, makes it impossible to use this 
means for heating the steel in any case where the cutting edges 
of the tools cannot be ground off to a sufficient depth to entirely 
remove the soft outside portion. In the experiments made, the 
steel was heated to a temperature of about 2100 degrees F. 

It was thought that possibly some other factors besides the 
barium chloride, which had been in use for a number of days, 
were the cause of the soft scale found on the tools. New barium 
chloride was, therefore, melted in the furnace, and a new set 
of tests made. In these tests the results obtained in the first 
series were entirely duplicated. During the first two days the 
steel hardened had no perceptible soft film or scale; but when 



144 HEAT-TREATMENT OF STEEL 

the bath had been in use for about three or four days there was 
a pronounced soft scale, and after a week the scale had a thick- 
ness practically the same as in the first series of tests. 

While, as already mentioned, no indications of the presence 
of ferro-tungsten were found in the bottom of the electric furnace, 
a soft, dark gray precipitate was found in the bottom of the 
electric furnace pot when it had been run with the same bath for 
about a week. The electrodes of the electric furnace are made 
of soft iron, and it is likely that the precipitate found originates 
from the electrodes, as it proved to be a metallic substance simi- 
lar to iron, and soft enough so that it could be easily filed, which, 
of course, would not have been the case had the precipitate con- 
sisted of ferro-tungsten. 

Another interesting fact has also been noticed in connection 
with the electric furnace. The amount of barium chloride used 
in a given time is much greater in an electric furnace when the 
bath is heated to a given temperature, than it is in a gas furnace 
with a bath at the same temperature. For some reason the 
electric current passing through the bath seems to make it easier 
for the molten salt to volatilize. 

The makers of electric hardening furnaces may undertake 
experiments which will give more complete data relating to 
the action of barium chloride in an electric furnace than those 
brought forth in the foregoing. At present, however, it seems 
that barium chloride is unsatisfactory for hardening high-speed 
steel in an electric furnace, and that it has only a compara- 
tively limited application when used in crucibles and heated in 
a gas furnace. Some users of barium chloride baths for heating 
tools for hardening have, as mentioned, entirely abandoned its 
use after several attempts to make it successful, while others still 
continue its use for parts and tools on which a thin soft scale is 
not objectionable and can be removed by grinding. 

It should be noted that in speaking of an electric heating 
furnace, only those furnaces using a barium chloride bath for 
the heating medium have been referred to. Other electric heat- 
ing furnaces are made, in which no heating bath is used, but the 
steel is heated in the air between two electrodes. This type of 



ELECTRIC HEATING FURNACES 1 45 

electric heating furnace is in a class by itself, and while it presents 
some difficulties, they are of a minor nature and do not come 
under the head of the present investigation. 

It should also be understood that the electric hardening 
furnace, using a barium chloride bath, may have a field of use- 
fulness for heating ordinary carbon steel tools for hardening, 
as in this case hardly any of the objections mentioned in the 
foregoing, and which apply to the hardening of high-speed steel 
only, are present, except the increased consumption of barium 
chloride and potassium chloride, with which latter salt the 
barium chloride must be mixed when used for heating carbon 
steel. This mixture is necessary in order to obtain the lower 
melting temperature of the bath required for carbon steel. 
The advantages of the electric hardening furnace for carbon 
steel tools are the greater rapidity with which they can be heated 
and the clean white surface on the tools thus hardened. The 
cutting edges of the tools seem to be protected by the coating 
from the heating bath which falls off when the tools are dipped. 
When dipping in an oil bath, this coating is not entirely re- 
moved, but it always disappears when dipping in hot water or 
soda solution. 



CHAPTER VIII 

MISCELLANEOUS TYPES OF ELECTRIC FURNACES IN 
GENERAL USE 

The growing demand for a higher quality in heat-treated steels 
and for a constant improvement of the physical properties of 
the metal is making the electric furnace a commercial necessity, 
both in the manufacture of the metal and in its hardening and 
tempering. Efforts are also being made to use electric furnaces 
in the fabrication of steels, and such furnaces are now in use for 
heating steel before it is drop-forged and bent to shape for leaf 
springs. This is due to the fact that when so heated impurities 
which are injurious to steel are either removed from or not al- 
lowed to penetrate the metal, and these impurities are thus 
present in lower percentages than when the steel is heated in 
furnaces that use a flame to generate the heat. Another reason 
is the ease with which the temperature can be controlled and, 
consequently, the greater accuracy that can be obtained in secur- 
ing the correct degree of temperature that is required for heat- 
treating or refining the steel. This has caused many different 
types of electric furnaces to be developed for heat-treating steels 
and several different principles are utilized in their design and 
construction. Of these, the Hoskins type was described in 
Chapter VI, and consequently will not be considered here. 
Most of the remaining types use either the arc or resistance prin- 
ciple for supplying the necessary heat to the oven or liquid bath 
furnace. 

A recently developed type of electric furnace is the Baily fur- 
nace, built by the Electric Furnace Co. of America, Alliance, 
Ohio. In one plant where this furnace is installed it is used 
in the manufacture of leaf springs. In this instance the opera- 
tion consists of inserting a leaf in the furnace and bringing it 
up to the fabricating heat, which must be 1800 degrees F. to 

146 



ELECTRIC HEATING FURNACES 147 

make it bend properly to shape. The leaf is then removed 
and bent, after which it is replaced in the furnace, brought up 
to the hardening temperature and then taken out to be quenched 
in oil contained in a tank. After that, it is again placed 
in the furnace where the oil is burned off to get the drawing 
temperature. 

It can be seen at a glance that this is a misuse of the electric 
furnace, as it ehminates its greatest point of excellence, namely, 
the accuracy with which the temperature can be controlled. The 
correct hardening temperature of carbon spring steels is about 
1500 degrees F. To reinsert the leaves in a furnace, the temper- 
ature of which is maintained at 1800 degrees, or more, and get 
them all within 150 degrees of the correct hardening temperature, 
during a day's run, is an impossibility. Occasionally one leaf 
may be taken out at the right temperature, but no means have 
been devised for telling when the whole length of a leaf has reached 
a temperature of 1500 degrees F., when it is in a furnace heated 
to 1800 degrees. Man's eyesight is not sufficiently delicate or 
accurate. Burning the oil off to obtain the drawing temperature 
is not to be spoken of seriously. If a separate furnace were used 
for the hardening operation, the heat in the oven could be main- 
tained at the correct temperature, which would be 1500 degrees 
in this case. The steel could be allowed to remain in the oven 
of the furnace until it had reached a uniform temperature and 
then be taken out for quenching. Pyrometers could be used to 
measure the temperature of the furnace oven, which would also 
be the temperature of the steel, and a more uniform product 
would be obtained. Another furnace operated at the correct 
drawing temperature would insure uniformity for that operation. 
The springs would then be much stronger and more resilient. 
The work could be turned out much faster in this way and with 
cheaper labor, which would lessen the cost of production. 

The furnace is supplied with all the accessories that are neces- 
sary for obtaining accurate temperatures and maintaining them 
during any period of operation. With these accessories the 
voltage can be varied to control the number of kilowatts of power 
supplied to the furnace, thus regulating its temperature. The 



148 HEAT-TREATMENT OF STEEL 

switchboard holds an ammeter, or current indicator, to show the 
amount of current that is flowing at any given time; a watt- 
meter to show the amount of power that is consumed per hour, 
day or month; a voltmeter to show the voltage of the current; 
and a recording pyrometer that shows on the chart the temper- 
ature the furnace has attained at any time during the day. 
These charts can be filed away to keep a permanent record of 
the temperature of the furnace on any work that is turned out. 

No flames, smoke, gases, fumes or roar comes from this fur- 
nace; therefore, no chimneys, piping or blast are needed and an 
air blast or steam is not required, as in the case of furnaces that 
use a flame. The temperature of the room is also lowered, as 
most of the heat is confined inside the furnace. Thus, the work- 
ing conditions are made so much more pleasant that the oper- 
ators are able to produce more springs for a day's work. 

This furnace is called a 100-kilowatt type, which means that 
its maximum consumption of power is 100 kilowatts per hour. 
The heating chamber is 5 feet wide by 7 feet deep by i| foot 
high and 600 pounds of steel plates can be heated to 1650 degrees 
F. in one hour. With all openings closed, the furnace can be 
maintained at this temperature with about 20 kilowatts of 
power, which replaces the heat lost through the walls, this result 
being obtained by adjusting the switch so that 200 amperes at 
100 volts will be flowing into the furnace. It is based on the 
theory that each kilowatt liberates 3412 B. T. U. in the furnace. 
If the doors of the furnace are left open, it will require another 
8 kilowatts to replace the heat lost through them, and if there 
are any holes in the sides or roof of the heating chamber for the 
heat to leak out, it would require considerably more power to 
keep the furnace up to the desired temperature. Therefore, it 
is important that no openings should be allowed to remain in 
the walls, and as the furnace can be completely relined with 
silicon-carbide, at a cost of some $2 or $3, there is no excuse for 
leakages to occur. 

With the preceding data, it is a simple matter to calculate 
the amount of current required to heat a given amount of steel. 
To heat one pound of steel to 1650 degrees F. requires 300 B.T.U.; 



ELECTRIC HEATING FURNACES 



149 



therefore to heat 600 pounds every hour would require 180,000 
B. T. U. per hour. This result divided by 3412 gives 53 as the 
number of kilowatts per hour that the furnace requires to gen- 
erate this amount of heat. If we add 20 kilowatts for the wall 
loss and 8 kilowatts for the loss through the door, we have a 
total of 81 kilowatts per hour as the amount of power that is 
needed to heat 600 pounds of steel per hour. This can be ob- 
tained by adjusting the regulating switch to give 500 amperes at 
160 volts. To leave the switch at this adjustment with the fur- 
nace empty would run its temperature up too high, so that if it 



r""r 





~~~~^\j 


s 


_ —^^^_^_^^_^ 







Machinery 



Fig. 1. 



Diagram showing the Principle on which the Baily 
Furnace operates 



were required to maintain a temperature of 1650 degrees F., 
while the furnace were standing idle for a short time, a readjust- 
ment of the switch would be made to give 200 amperes at 100 
volts to make up the wall loss of 20 kilowatts when the doors 
are closed. If the doors are left open, enough more power would 
be required to make up the 8 kilowatts loss through that opening. 
A 60-kilowatt furnace of the same type was built for a forge 
shop in Alliance, Ohio. This furnace is operated every day to 
heat chrome-vanadium steel to a forging temperature of over 
2000 degrees F., and allowed to get cold at night. It has a 
capacity of 300 pounds of steel per hour and is of the same type 
as the heat-treating furnaces. In fact, it could be operated just 



IS© 



HEAT-TREATMENT OF STEEL 



as efficiently at the hardening temperatures of the carbon or 
alloy steels. 

The principle on which these furnaces operate is shown by the 
diagram, Fig. i. Here T represents the heating chamber and 5 
the opening or door leading into it, while P is the floor of the 
heating chamber on which the work is placed that is to be heat- 
treated. The leads which conduct the current to and from the 




Machinery 



Fig. 2. Electrically Heated Salt-bath Furnace showing Method of 
starting with a Hand Electrode 

furnace are shown at L and the copper plates that conduct it to 
the electrodes N are shown at K. A channel O running under- 
neath the floor of the heating chamber is filled with a resistance 
material, which in this case is coke ground to about the size of 
peas. The electrodes N extend several inches into the ground 
coke, and as the electricity passes through the resistance material, 
from one electrode to the other, the required heat is generated. 
As many channels may be used as the size of the heating cham- 
ber makes necessary. 



ELECTRIC HEATING FURNACES 1 51 

The Salt-bath Furnace. — The salt-bath furnace illustrated 
in Fig. 2 is of the type already described in Chapter VII. The 
steel is immersed in a molten metal salt to bring it up to the 
proper hardening or drawing temperature. The noteworthy 
feature of this furnace is that it is electrically heated, and such 
a furnace can be made to last indefinitely. It is best to con- 
struct a sheet-steel shell A that is held together with angle irons, 
lined with one inch of asbestos and then built up with about 
12 inches of common brick B on the bottom and four sides. 
This brickwork should again be lined with one inch of asbestos C, 
inside of which about 8 inches of firebrick D is laid on the bottom 
and four sides. When completed, the pot so formed should be 
of the right size for ordinary classes of work. It ought to be 
enough larger than the steel to be heated to allow space for two 
electrodes E of boiler plate on opposite sides of the pot, as these 
are placed inside of the salt bath, and also to allow for keeping 
the steel at least one inch away from the bottom, sides and 
electrodes, and immersed two inches below the top of the bath. 
When such a furnace has cooled down, the hard salt is not a good 
conductor of electricity. Therefore in starting up, it is neces- 
sary to chip a channel across the top of the salt, lay a ^-inch 
round carbon F in the channel, cover it with the salt chips and 
start melting them with a hand electrode, as shown in the illus- 
tration. When the channel is filled with molten salt, from one 
boiler plate electrode to the other, it will start the electric current 
flowing and the balance of the salt in the bath will soon become 
molten. The temperature is raised and controlled in practically 
the same manner as with the oven furnace previously described. 
The electrodes will last something like 3000 hours at temperatures 
that are high enough for hardening carbon steels. 

As mentioned in the preceding chapter, when steel is heated to 
the hardening temperatures in a salt bath and then removed to 
be quenched, a thin coating of the salt adheres to the steel and 
prevents oxygen from attacking it and forming an oxide on its 
surface or raising a scale, while it is passing through the air to 
the quenching bath. This adhering salt cracks off when it is 
suddenly chilled in the quenching bath and leaves the steel with 



*52 



HEAT-TREATMENT OF STEEL 



a natural color instead of with the black appearance that is pro- 
duced when hardening it in other ways. At the higher harden- 
ing temperatures of high-speed steels, the salt bath furnaces 
that use graphite crucibles cause the tools heated in them to be- 
come pitted. In this electric furnace, however, the pot that 
holds the salt bath can be built up of other refractory mate- 
rials, such as silicon-carbide and electrically calcined magnesia, 
and then no pitting occurs at any of the temperatures used for 
hardening. 

Many different kinds of salts have been experimented with 
and the best kind to use for the bath depends on the temperature 

Melting Points of Different Salts Used For Heat-treating Steel 



Name of Salt 


Melting 
Temp., 
Deg. F. 


Name of Salt 


Melting 
Temp., 
Deg. F. 


Barium chloride 

Sodium chloride 

Potassium chloride 

Calcium chloride 

Magnesium chloride 

Lithium chloride 

Lead chloride 


I58o 

1418 

1346 

1328 

1306 

III2 

932 

928 

844 

813 

572 

504 

356 


Potassium carbonate 

Sodium carbonate 

Lithium carbonate 

Potassium nitrate 

Sodium nitrate 


IS26 

1317 

1283 

644 

572 

113 

1832 

1832 

1664 

1656 

1474 

1454 

i35o 


Sodium silicate 

Barium fluoride 

Calcium fluoride 


Cupric chloride 


Silver chloride 


Magnesium fluoride 

Sodium fluoride 


Cuprous chloride 

Ferric chloride 


Lithium fluoride 

Potassium fluoride 

Strontium fluoride 


Zinc chloride 


Aluminum chloride 



that is required. The melting points of the various salts that 
have been used are given in the accompanying table. Of the 
salts specified some are too expensive for commercial work, others 
volatilize too easily and still others cannot be used for various 
reasons. Several combinations can be made that are better 
than when one salt is used alone, and some of these combinations 
have a lower melting point than either of the salts forming the 
mixture. For temperatures between 1800 and 2400 degrees F., 
chemically pure barium chloride is without doubt the best salt 
to use, as it volatilizes less than any of the others and is low 
in price. For temperatures between 1400 and 1650 degrees F. 
three parts of barium chloride to two parts of potassium chloride 



ELECTRIC HEATING FURNACES 1 53 

give excellent results. For any of the drawing temperatures 
below 1075 and above 480 degrees F. equal parts of potassium 
nitrate and sodium nitrate make a salt bath that is satisfactory 
in every way. This can be kept molten at 400 degrees F. if it 
is continually stirred, but if left standing it will solidify at about 
475 degrees F. 

When the proper salts are used there is very little loss from 
volatilization and aside from the cost of current, the expense of 
operating this furnace is very slight. It is particularly adapted 
for treating small work that can be loaded into metal baskets 
or racks and then immersed in the molten salt, as large quantities 
can be handled in this way and the work is clean when finished. 
In one case, 70 pounds of safety razor blades are heated to the 
hardening temperature in this way. They are next quenched 
and then reheated to the drawing temperature in another elec- 
trically heated salt-bath furnace. Both furnaces are main- 
tained at the correct temperatures for the hardening and temper- 
ing operations by a switch with eleven points that correspond 
to ten steps either way and thus gives twenty-one adjustments. 
A pyrometer tells when the bath is at the correct temperature, 
and then it is only a question of leaving the work in the bath 
long enough to reach a uniform temperature. The thickness 
of the work determines the length of time it should be kept 
immersed, and with a little experience this can be definitely 
determined. 

Electric Arc Heating. — The electric arc has been successfully 
used for heating steel to hardening temperatures, and it is espe- 
cially applicable for localizing the hardening in a certain part of 
the piece, the point of a cutting tool being a notable example. 
In Fig. 3 is shown a home-made apparatus that was rigged up 
for this purpose. The barrels filled with salt water take the 
place of a more expensive transformer and rheostat. By raising 
and lowering the steel terminal plates A in the barrels, the elec- 
tric current can be controlled so that the tool is heated to the 
desired temperature. The carbon holder B is a very simple 
thing to make and any cast-iron plate C can be set on a rubber 
mat D to hold the tool E to be heated. A welding heat can also 



■54 



HEAT-TREATMENT OF STEEL 




ELECTRIC HEATING FURNACES 155 

be obtained with this apparatus. It is essential that all parts 
of the body be protected from burns, as the heat from the arc 
will burn any exposed part in the same way that a sun glass burns 
on a hot day. Therefore it is necessary to wear gauntlets and a 
face shield with colored eye-glasses. 

The steel is only heated in a spot directly under the carbon, 
and to heat the desired surface it is necessary to keep the carbon 
moving in a circle. It should not be brought too near the cutting 
edge of the tool and the arc should be started at a very low volt- 
age, which is steadily increased to the desired point by adjusting 
the shunt rheostat. The carbon must also be kept a short dis- 
tance away from the steel, for if it touches it is very likely to 
melt the metal at that point. The correct hardening temper- 
ature must be judged with the eye, as it would be very difficult 
to measure it with any kind of an instrument. 

As steel becomes non-magnetic when it reaches the correct 
hardening temperature, or the transformation point, a magnet 
might be used to ascertain when it had reached the non-magnetic 
stage; but as steel cannot be instantly cooled from the harden- 
ing to atmospheric temperature, it must be heated to from 25 to 
40 degrees above the non-magnetic point to allow for the lag 
when quenching. It is very difficult to judge these 25 to 40 de- 
grees with a magnet, and it cannot be used for accurate work in 
hardening. 

The rapidity with which a piece of steel can be heated to the 
hardening temperature is the greatest recommendation of the 
apparatus just described, as it takes only two or three minutes 
to heat quite a large surface on a fairly thick piece. If it were not 
heated and quenched quickly, the oxygen in the air would have 
time to raise quite an oxide or scale on the heated steel. If this 
principle were used regularly, it would be much better to fit the 
apparatus with a transformer and switchboard control in place 
of the water barrels, as these require constant attention to keep 
the water from boiling over, and to raise and lower the terminal 
plates. 

The most rapid method of hardening steel is doubtless that 
in which the furnace shown in Fig. 4 is used. This consists of 



156 



HEAT-TREATMENT OF STEEL 



a cast-iron tank containing a potassium carbonate solution, a 
clamp in which to hold the piece to be hardened, and a rheostat, 
switches, fuses and wiring. After clamping the tool, it is only 
necessary to turn on the current, lower the tool into the solution 
and, when it has attained the proper temperature, turn off the 
current and allow the steel piece to be quenched by the solution. 
When the steel enters the potassium carbonate solution, it com- 
pletes the circuit and immediately begins to heat up. The 
correct temperature is reached on a good-sized piece in about a 
minute and, being quenched before it is removed from the bath, 



i V ■ 

I I _ r~--TOOL CLAMP 



-a- — -tb — sfo 



jool to be. 
""hardened 



TQpOOOOOOO. 



-POTASSIUM 

"CARBONATE 

SOLUTION 



FUSES / 




8H.UNT RHEOSTAT 



//////////////////////////7 



Machinery 



Fig. 4. A Rapid Method of Hardening Steel 

nothing can attack the steel to discolor -it. Consequently, when 
taken from the bath it has a clean steel-colored appearance. 
Being heated uniformly on all sides, there is no tendency for the 
work to warp or become distorted, and it can be lowered into 
the bath only as far as it is desired to have it hardened. With 
a little experience, the rheostat can be set at the point that will 
heat the steel to just the required temperature, and then it is 
only a question of leaving the steel in the bath long enough to 
attain this temperature before turning off the current. For 
permanent use, this apparatus can be fitted up to enable the 
current to be quickly regulated for obtaining any desired tem- 
perature. 



CHAPTER IX 
MISCELLANEOUS HARDENING METHODS 

Pack-hardening. — Pack-hardening, as the term is generally 
understood, consists in treating steel, generally tool steel, with 
some carbonaceous material until it will harden in oil. It is 
well known that steel hardened in oil is less liable to spring than 
when hardened in water. The tendency to crack is almost 
entirely done away with, unless the steel is improperly treated 
in the fire, and the maximum of toughness is procured in the 
hardened parts. Now, if we are able to treat the steel so that 
it will be as hard as though dipped in water, and yet have the 
toughness due to oil-hardening, and at the same time reduce 
the tendency to spring to a minimum, it would seem that we 
have the ideal method of hardening. 

Packing Materials. — The process consists essentially in sup- 
plying the surface of the steel with an additional amount of 
carbon by some material that will not in any way injure the steel. 
In order to provide the additional carbon, the steel must be 
packed in iron hardening boxes with the carbonizing material. 
Some have used charcoal for this purpose. While charcoal is 
a carbonizing agent, and is used frequently in casehardening 
machine steel, yet its effect on high-grade steel in the process of 
carbonizing is not satisfactory, as it renders the steel coarse, and 
very similar to blister steel. No form of bone should be used 
when pack-hardening tool steel, as bone contains a high per- 
centage of phosphorus, and the effect of this is to make steel 
weak and brittle. 

For steel that does not contain more than 1.25 per cent carbon 
(125 points), charred leather gives the best results. Above this 
percentage use charred hoofs, or horns, or a mixture of the two. 
The leather, hoofs, or horns, may be used over and over by add* 
ing a quantity of new material each time. 

157 



158 



HEAT-TREATMENT OF STEEL 



Method of Packing. — The work should be packed in the 
hardening boxes so that no part of any piece of work comes in 
contact with the boxes; in fact, there should be at least | inch 
space between the work and the box. A layer of the carbonizing 
material should be placed in the bottom of the box, and a layer 
of work placed on this, taking care that no two pieces touch 
each other. If we are treating gages, or pieces of steel that are 
apt to spring unless care is used, we should make sure that they 
are so placed in the box that there will be as little liability of 
springing as possible when they are drawn up through the pack- 
ing material. They must not be dumped into the hardening 
bath, as is the case when ordinary casehardening is done. 




Fig. i. Piece to be Hardened and Wire for Handling It 

In order to be able to properly handle the work, each piece 
should be wired with a piece of iron binding wire, as shown in 
Fig. i, and the pieces so placed in the box that there will be the 
least resistance possible when drawing them out. At times they 
may stand on edge, as shown in Fig. i. For certain shapes, 
however, it is advisable to stand them on end. 

When several layers of work are packed in a box, the wires 
should be so arranged around the edge of the box that the va- 
rious layers may be taken out in order, commencing with the 
top row. This is easily accomplished by marking the sides of 
the box with chalk, designating the side where the top row of 
wires is, as i, the one where the second row is, as 2, and so on. 



PACK-HARDENING 



159 



Unless we adopt some such method, the pieces get all mixed up, 
and some will be drawn to the surface of the packing material, 
and will cool before the operator has a chance to dip them. 

Heating the Steel. — As in all heat-treatment of tool steel, 
the heat should be as low as is consistent with desired results, 
and the heat must be uniform throughout the box. It is also 
necessary that we gage the length of time the steel is exposed 
to the action of the carbonaceous material. Unsatisfactory 
results follow any attempt to gage the length of time by the 
time the boxes are in the furnace. In order that the operator 
may know when the contents of the box are heated, holes are 
drilled through the cover of the box at the center, and test wires 



TEST WIRE 






COVER 


\ 






/ 


1 







Machinery 



Fig. 2. Diagrammatical View of Hardening Box with Test Wires which 
enable the Operator to determine the Temperature within the Box 

are run down to the bottom of the box, as shown in Fig. 2. These 
wires should project about one inch above the top of the cover. 
The holes in the cover may be of any size to accommodate the 
wire to be used; a good size is J-inch hole for ^g-inch wire. 
When the box has been in the fire, according to the judgment 
of the operator, until the contents are heated to a low red, a wire 
may be drawn, by means of long tongs, and its condition noted; 
if it is red hot, begin timing the heat; if it is not red, wait a 
little while, and draw another. Continue doing this until one 
is drawn that is of the desired temperature. The wires passing 
down at the center of the box, and between the pieces, will not 
be red until the pieces are of the same temperature. 

The length of time necessary to expose the pieces to the action 
of the heat depends upon how deep we wish to harden the steel. 



i6o 



HEAT-TREATMENT OF STEEL 



For ordinary snap gages i| to 2 hours after the steel is red hot 
is sufficient, but the time must be varied according to the per- 
centage of carbon that the steel contains and its intended use. 

Pack-hardening Gages. — Sometimes locating gages are made 
with the gaging holes made to the finished size of the gage. 
This method is not to be advocated where it is possible to use 
hardened bushings. In the latter case, the holes in the gage 
may be made of the proper size for the bushings, and the gage 
left soft, while the bushings are hardened, ground and lapped, 
and pressed into place without any tendency to distort the 
gage. But when it is necessary to make the gage of one piece, 
and have the gaging holes to size in the gage without bushings, 













«««M««§^^ 






^HARDENED SURFACES 


/ -««■ 


«««i««««M 






HANDLE, LEFT SOFT 














Machi 


nery 



Fig. 3. A Gage Hardened on the Surfaces Indicated 

then the pack-hardening method will be found to work satis- 
factorily, as the heat may be very low, and the tendency to dis- 
tortion will be eHminated, provided the processes of annealing 
and machining have been properly done. 

As an example of pack-hardening gages, a case from practice 
may be cited. The gage was of the form shown in Fig. 3, and 
it was necessary that the walls of the opening through the gage 
be hard, yet the opening must retain its shape. The hole was 
rilled with finely-pulverized charred leather; the handle and the 
portion connecting it with the body were covered with fireclay 
which was wound with fine iron binding wire to prevent it falling 
away when baked. The gages were packed in scale collected 
in the forge shop. This scale came from the outside of pieces of 
iron and steel as they were being forged. Being free from car- 



PACK-HARDENING 



161 



bon, they absorbed or took the carbon from the surface of the 
steel. The ends of the opening through the gages were covered 
with fireclay mixed with water to the consistency of dough, 
which was allowed to harden before the gages were packed. The 
fireclay prevented the carbon gas escaping from the leather, as 
the scale would have taken it up very quickly. 

When the gages had been exposed to the carbonizing influence 
of the leather for one and one-half hour, they were removed from 
the box in which they were heated, placed in a bath of raw lin- 




Fig. 4 . 



Receiving Gage of a Type which is Hardened to Advantage 
by the Pack-hardening Method 



seed oil, one at a time, and a jet of oil was pumped through the 
opening after the leather had been removed. The fireclay around 
the handle and the portion connecting it with the body was left 
on until the gage was hardened, when it was removed. The walls 
of the hole were found to be hard, and as the surface of the gage 
was practically decarbonized, there was little danger of its pull- 
ing the piece of steel out of shape. The handle and adjoining 
portion, being protected by the fireclay, did not harden, or even 
cool quickly enough to distort the gaging portion in any way. 
Often receiving gages are made of several pieces which are 



1 62 HEAT-TREATMENT OF STEEL 

fastened to a plate as shown in Fig. 4, which is a receiving gage 
for a gun hammer, and it is necessary that the various portions 
be gaged accurately, and that each portion bear a certain rela- 
tion to every other portion. 

As shown, the various portions of the gage are made in sections, 
fitted in place and hardened. Unless these pieces are hardened 
by some method that eliminates the tendency to spring, they 
will be of little use after they are hardened. This is a case for 
pack-hardening. Pack the pieces in leather in a small iron box, 
run for one hour after they reach a low, red heat, and harden in 
raw linseed or sperm oil. It will not be found necessary to heat 
the steel treated in this way as hot as if heated in an open fire 
and dipped in water. It is not necessary to heat steel in the 
form of gages quite as hot as if it were made into cutting tools; 
but, even in the latter case, be sure to keep the heat down, and 
do not dip in extremely cold oil; have it warm, but not hot. 

Pack-hardening Hammer Dies. — The hardening of large die- 
blocks, for either hot or cold work, is a serious problem to most 
manufacturing concerns, because, in addition to the cost of the 
steel, a very heavy expense for the labor of die-cutting is in- 
volved. The total cost of a pair of dies frequently ranges from 
one hundred to three hundred dollars or more, so that the loss 
of a single die in hardening is a serious item of expense, aside 
from the long time required to replace it. Furthermore, a ham- 
mer die that is subjected to the blows of a drop, weighing from 
twelve hundred to eighteen hundred pounds, must be very skill- 
fully heat-treated in order to stand up under the repeated heavy 
shocks without breaking. 

The requisites for a properly hardened hammer die are: a 
perfectly hard face, with a sufficient depth of hardening to prevent 
the surface from sinking; sharp, clean edges and corners; a 
soft, tough body to insure against breakage ; freedom from warp- 
ing or change of size, due to shrinkage of the steel in cooling. 
The face of the die should not only file hard, but should stand 
a smart blow of a hardened steel hammer without perceptible 
dent. With any good uniform die steel these qualities may be 
readily obtained by the following treatment. 



PACK-HARDENING 163 

Furnaces for Heating. — For heating the dies, an oil or gas 
furnace is the ideal equipment, but a coal or coke-fired muffle, 
capable of maintaining a temperature of at least 1600 degrees F., 
will answer the purpose, provided the temperature can be held 
constant. The great advantage of the gas or oil furnace over 
the coal or coke-fired muffle is the greater rapidity with which 
the temperature can be raised or lowered, and the ease with 
which it can be held constant. 

A temperature-indicating device, either pyrometer or pyro- 
scope, is an absolute necessity if good results are to be obtained, 
as it is impossible to judge the temperature by the eye with suffi- 
cient accuracy for hardening this class of dies. A temperature 
difference of twenty-five degrees has a very noticeable effect 
upon the die, while a fifty-degree variation may spoil it entirely, 
particularly if at the critical high heat demanded with some of 
the best alloy steels. Under certain conditions, a temperature 
difference of even seventy-five degrees cannot be detected by 
the eye with certainty. No guesswork should be permitted to 
enter into the hardening of dies, as a few failures cost many 
times the price of a good pyrometer or pyroscope. 

Packing the Dies. — Dies should always be packed for hard- 
ening. To pack a die properly, mix a thick paste of linseed or 
cottonseed oil and powdered bone-black. Paint the face of the 
die with a thick coat of this paste, which will bake on under the 
action of the heat and protect the delicate edges from oxidation 
through contact with the air. 

Next, take a shallow sheet-iron or cast-iron box, an inch or 
so wider than the die all around, and fill the bottom to the depth 
of an inch with fresh bone and powdered charcoal, mixed half- 
and-half. Place the die, face down, in the center of the box, 
taking care not to displace the paste on the face. Now fill in 
between the sides of the box and the die with more bone and 
charcoal, mixed half-and-half, right up to the top of the box. 
The die should set in the box only deep enough to allow its top 
edge or back to project about an inch above the top of the box, 
for convenience in handling with the tongs when removing from 
the box to quench. Now cover the space between the upper 



1 64 



HEAT-TREATMENT OF STEEL 



edge of the box and the sides of the die with a thick layer of wet 
clay paste, which will bake on and keep the charcoal from burn- 
ing out, and the die is ready for the furnace. Fig. 5 shows the 
arrangement of the die and packing in the box. 

Method of Heating. — Die steels in general give the best 
results when quenched at the temperature which produces the 
densest, finest-grained fracture. Unless this temperature is 
exactly known for the steel used, first ascertain it by placing a 
number of small pieces of the steel in the furnace (they do not 
need to be packed for this purpose) and hardening and fracturing 




DIE BLOCK 




CLAY PASTE 



SSI ,: §S?S^^iS^SH-^^sS^^> , ^£^^B < _! 



BONE AND CHARCOAL PACKING 



Machinery 



Fig. 5. Packing a Die-block for Heating 



them at different heats, starting at a low heat and gradually 
working up until the greatest refinement of the grain of the steel 
is obtained. Note the temperature at which it is obtained, as 
that will be the correct quenching heat. 

The dies should be put into the furnace as soon as it is lighted 
and allowed to " come up " with it. If the correct quenching 
temperature for the steel is, say, 1500 degrees F., then the fur- 
nace should be checked when the pyrometer indicates 1400 de- 
grees, and the dies allowed to soak at that even heat for three 
or four hours. Then the heat should be slowly raised to the 
final 1500 degrees, and held at that point for one or two hours 



PACK-HARDENING 165 

longer, according to the size of the die, large blocks requiring 
a longer time to attain an even temperature throughout than 
small ones. Then the die is ready for quenching. 

Do not attempt to hurry the heating of dies, as it takes several 
hours for the heat to penetrate the packing and thoroughly soak 
into the die-block, until the outside and center are of the same 
temperature. More dies crack in cooling, or afterwards, from 
insufficient heating than from any other one cause. With this 
in mind, five hours is the least permissible total time in the fur- 
nace, and seven to eight hours is much safer. Most expert die 
hardeners put their dies into the furnace in the early morning, 
and take them out in the late afternoon. 

Care must also be taken that the dies get an even heat in 
the furnace. As may readily be proved by manipulation of the 
pyrometer rod, some parts of a furnace are frequently much 
hotter than others; so, to avoid getting one end of the die hotter 
than the other, it should be occasionally reversed, end for end, 
while heating. 

Methods of Cooling. — The proper cooling of dies is as essen- 
tial to good results as the heating itself. Even when a die has 
been correctly heated, improper cooling may make it too soft, 
or warp it, or even cause it to crack, rendering it totally unfit 
for use. The requisites for properly cooling a die are : a sufficient 
flow of cold water to cause it to harden to the necessary degree 
of surface hardness to withstand wear, and to a sufficient depth 
to prevent the surface from sinking under heavy blows of the 
hammer; and means for applying the water in such a manner 
as to cause the least possible warping. The rate of cooling has 
an appreciable effect upon the initial heat required to make a 
die hard enough. If the water equipment permits of very quick 
cooling, a somewhat lower heat may be used, and the lower the 
heat, the less trouble from warping. It is useless to attempt 
to cool a die-block of any appreciable size in a tank of still water, 
as the heat in the large body of metal draws the surface faster 
than the water can harden it. It will harden upon the immedi- 
ate surface, under such treatment, if heated enough, but there 
will be no depth to the hardening, and the face will soon sink in 



166 HEAT-TREATMENT OF STEEL 

use, causing cracks to form. A properly hardened die should 
halve a depth of hardening of from J to | of an inch, according 
to the steel used. Oil should not be used to harden hammer 
dies, as its cooling action is not sharp enough to produce a suffi- 
cient depth of hardening. 

In cooling, the face of the die should receive a sufficient flow 
of cold water to cause it to harden to the greatest possible depth. 
The back of the die should at the same time receive a similar 
treatment, to make the shrinkage of face and back equal, thereby 
preventing warping. The sides of the die may be left to take 
care of themselves. Dies may be hardened either face up or 
face down, according to the equipment used. 

Cooling Baths. — Fig. 6 shows a most desirable arrangement 
for hardening dies face down. A is the cooling tank; B, the 
die; C, a six-inch water pipe; D, an eight-inch overflow pipe; 
E, an inch-and-a-half water pipe; F, a quick-opening valve; 
K, a flattened nozzle with a long, narrow opening; G, a quick- 
opening valve; and /, adjustable rods for supporting the die 
at the two ends. 

To harden a die, the water level is lowered to the line H, 
by means of the two-inch drain I. The packing box contain- 
ing the die is then removed from the furnace and set upon the 
floor. The back of the die, which protrudes an inch or more 
above the packing, is grasped firmly with a pair of tongs and 
lifted from the box. If the bone paste, which has baked on in 
a hard crust, sticks to the face of the die, no attempt is made 
to remove it, as it serves to keep the air from the face of the 
die until it is under the water, at which time the paste will crack 
and fly off. The die is immediately placed in the tank, face 
down, under the elongated nozzle K, the two ends resting upon 
the adjustable supports /. 

When in this position, the die should be central over the pipe 
C, as well as central under the nozzle K. As soon as the die is 
in place, the valve G is turned on, which permits a large volume 
of water under pressure to flow out of the pipe C up against the 
face of the die. This flow of water rapidly raises the level in 
the tank from the line H, so that in the course of a few seconds 



PACK-HARDENING 



167 



the whole die-block is immersed. Just before the water level 
reaches the top of the die-block, the valve F should be quickly 
opened, permitting a small stream of water at high pressure to 
strike the back of the die, thus equalizing the cooling strains. 
When the water reaches the level L, an inch or two above the top 
of the die, the overflow pipe D takes care of it and prevents it 



i 




\ 










i t r\ \ 1 


1 
1 




^~" 




I 




1 



PLAN OF DIE SUPPORTS 





Machinery 



Fig. 6. Cooling Bath Arranged to throw Water against both Faces 
of the Die 



from rising further. The flow from pipes C and E is allowed 
to continue until the die-block is cool enough to handle, which 
requires only about two minutes from the time the water is turned 
on, for a medium-sized die. This is very rapid cooling, as this 
same die-block, if put in a tank of still water to cool, would re- 
tain a visible red for five or six minutes. The application of the 



i68 



HEAT-TREATMENT OF STEEL 



water to both face and back of the die, equally, balances the 
cooling strains and does away with warping. 

As it is not always convenient or possible to get a six-inch 
water supply even at low pressure into the hardening room, an 
alternative scheme is shown in Fig. 7. This is a method for 
hardening the die face up, and requires only a small water supply, 
an inch-and-a-half pipe at forty to fifty pounds pressure, such as 
is obtainable in most towns. With this arrangement we have 



h 




Machinery 



Fig. 7. Cooling Bath Arranged to partially Immerse the Die and 
to throw Water against the Top Face of the Die 

the tank A, the iron plate B, supporting the die-block C, the 
inlet pipe D, fitted with the nozzle E, and the overflow pipe F. 
In operation, the water is lowered by means of the drain / 
to the level /, which covers the plate B about an inch deep. 
The die is removed from the furnace as before, and placed upon 
the plate B, centered under the nozzle E. The water that covers 
the plate, and the cold iron itself, rapidly cools the back of the 
die. As soon as the die is in place, the valve D is opened, and 
the water from the nozzle E strikes the face of the die. The 



PACK-HARDENING 169. 

water level in the tank rises rapidly to the level H, completely 
immersing the die, and the overflow pipe F holds it there. The 
water from the pipe D is permitted to flow upon the face of the 
die until cool enough to handle. 

With this last arrangement, the flow of the water from the 
pipe D must be so regulated as not to cool the face faster than 
the back of the die; otherwise warping will occur. With a 
little practice this method will produce nearly as good results 
as the double-pipe arrangement, except that the dies will have 
a little greater tendency to warp. Either method, though, is 
far ahead of the common tank in this respect. 

Tempering in Oil. — Dies should be tempered, or drawn, as 
soon as they are cool enough to remove from the tank. If not 
drawn at once, they are liable to develop cracks. For temper- 
ing, the proper equipment consists of a small tank of oil, a suit- 
able burner underneath for heating it, and a high-temperature 
thermometer. Any high-grade cylinder oil of high flash point 
is suitable for the work. Low-grade oils smoke unpleasantly 
at moderate heats, and will not stand high temperatures. The 
drawing temperatures of die steels range from 350 to 480 degrees 
F. The color barely starts at 350, while 480 produces a very 
full straw. 

Heat the oil bath to the temperature corresponding to the 
desired color, immerse the die-block completely in the oil, and 
let it soak for an hour or more. Remove from the oil, quench 
in water or cool oil, and place in a warm, even temperature over 
night, or until ready to use. A lead bath should not be used for 
dies, as the lead will stick to the intricate corners and angles and 
cause trouble. Oil is the only proper medium for this purpose. 

Do not attempt to draw die-blocks by simply starting the sur- 
face color on a hot plate or in a furnace, as is common practice 
in many shops. This heats only the immediate surface, with- 
out relieving the greater cooling strains that are locked into the 
body of the metal. For hammer dies under heavy service it 
is most important that these strains be thoroughly released; 
otherwise the dies are apt to crack or break down prematurely. 
Surface-drawn die-blocks should be classified with the methods 



TJO HEAT-TREATMENT OF STEEL 

of the old-fashioned spring maker who tempered his springs 
by rubbing them with a charred pine stick, the sparks given 
off by the burning wood presumably indicating the degree of 
heat. 

Points to Remember. — i. Fresh bone has a decided carbon- 
izing (surface hardening) effect upon the steel. The ordinary 
mixture is half bone and half charcoal. More bone gives greater 
hardness, more charcoal less hardness, for a given heat. The 
proportions of bone and charcoal should be varied to suit the 
work in hand. Increase of hardness means increase of brittle- 
ness. 

2. Unless the face of the die is treated with a jet of water under 
pressure, the sunken parts of the pattern will not harden equally 
with the face. When dipped into still water to harden, steam 
forms in the die cavity, which keeps the water from entering 
to properly harden these parts of the die. To overcome this, 
the water must be forced into the sunken parts of the die by 
pressure sufficient to overcome the resistance of the steam that 
is formed by the heat. 

3. The most skillfully hardened dies will not give good ser- 
vice unless set absolutely parallel as to their surfaces under the 
hammer. In a block that is set in the hammer or anvil a little 
" off " the strains created are enormous, and breakage is sure 
to result speedily. 

4. Hammer dies must have depth of hardening, as well as 
surface hardness, to withstand the heavy blows to which they 
are subjected. Other kinds of dies do not need this, because 
the work is light, and service is merely a matter of wear of 
their edges. Sinking of the face of a hammer die means the 
development of cracks every time, so the steels used should be 
selected for their deep-hardening qualities. This means alloy 
steels, generally, as few carbon steels will harden to a depth 
of more than T a g inch, which is entirely inadequate for this 
service. 

To Prevent Scale on Dies when Hardening. — Scale can be 
prevented from forming on dies when hardening, if the dies are 
dipped in water before they are heated and then put into dry 



PACK-HARDENING 171 

salt, letting all the salt that will cling to the dies remain on 
them. After this, the pieces are heated, and immersed in the 
quenching bath as usual. The scale or crust of salt will fall off 
in the bath and the piece so treated will have the appearance of 
a piece which has been heated in cyanide. 

Heat-treating Dies and Tools used in Forging Machines. — 
Vanadium alloy steel has been found very suitable for dies for 
forging machines. Two grades of vanadium tool steel are recom- 
mended for forging machine dies by the American Vanadium 
Co., of Pittsburg, Pa. One contains 0.50 per cent carbon; from 
0.80 to 1. 10 per cent chromium; from 0.40 to 0.60 per cent 
manganese; and not less than 0.16 per cent vanadium. The 
silicon content should not exceed 0.20 per cent. 

The heat- treatment recommended for this steel is as follows: 
Heat to 1550 degrees F. and quench in oil; then reheat from 
1425 to 1450 degrees F. and quench in water, submerging the 
face of the die only. When this method is used, the die is drawn 
by the heat remaining in the body and is thus tempered and the 
life increased. 

The second kind of vanadium steel recommended contains 
from 0.65 to 0.75 per cent carbon; from 0.40 to 0.60 per cent 
manganese; and not less than 0.16 per cent vanadium. The 
silicon content should not exceed 0.20 per cent. The heat- 
treatment for this steel should be as follows: Heat to 1525 
degrees F. and quench in water with only the face of the die 
submerged. The length of life of vanadium steel dies is stated 
to be about six times the life of dies made from ordinary high- 
carbon tool steel. 

When ordinary carbon tool steel is used for forging machine 
dies, these should be hardened as usual and the temper should be 
drawn so that they can be just touched with a file, or, in other 
words, to a light straw color. 

Heat-treatment for Vanadium Tool Steel. — For a quenching 
bath, either water, oil, brine or sulphuric acid may be used, but 
oil is preferred if the best results are to be obtained. W T hen 
using oil as a quenching bath, heat the steel to between 1450 
and 1550 degrees F. (dull cherry-red) and then quench in oil. 



172 HEAT-TREATMENT OF STEEL 

Draw the temper to about 425 degrees F. (very faint yellow). 
When using water as a hardening bath, heat to about 1400 de- 
grees F. and quench. 

Hardening the Heads of Forge Tools. — In the interests of 
safety and economy, it is advisable to harden the heads of all 
forge tools, such as sets, chisels, fullers, swages or any other 
tools made of high carbon steel, that are used by being struck 
upon their heads with a hammer. Upon the face of it, this may 
not appeal very strongly to a great many users of such tools, as 
it is common practice to leave the heads just as soft as possible. 
Some have even recommended annealing the heads from time 
to time to keep them soft, the idea being to prevent burrs break- 
ing off and flying around when the heads get battered down 
from use. This is a mistake, for when soft steel is subjected to 
hammering for any length of time, it not only begins to batter 
down and burr over on the edges but also becomes crystallized. 
This makes the metal very brittle — although it is quite soft — 
with the result that the burrs begin to break off and fly around 
when the tool is in use. This shortens the life of the tool. 

Experience has proved that high carbon steel can be hard- 
ened and tempered to make it suitable for any purpose from 
cutting tool steel in an annealed condition to withstanding the 
impact of blows. Therefore, theory, as well as the results of 
practical experience which will be given later, favors hardening 
the heads of tools. The best and safest type of head to use, 
either in a soft or hardened condition, is dome-shaped without 
a single sharp corner. 

For a number of years, a user of these tools has made a prac- 
tice of hardening forge tools on both ends with very good suc- 
cess. An idea of how these hardened tools will wear may be 
obtained from the fact that a few tools have been in constant 
use for upwards of eight years. A sledge nearly nine years old 
has been in use practically every working day since it was made 
and is still in perfect condition, not having even been reground 
on the face. A if-inch swage, which is about eight years old, 
has been in constant use the whole time and is still without per- 
ceptible signs of wear. A rectangular set, which is one of the 



HARDENING FORGE TOOLS 



173 



tools that a blacksmith uses the most, is about five years old, 
and like the sledge it is still in perfect condition. 

The method of hardening a sledge or hammer of any kind is 
to first heat the peen to as near the point of decalescence as pos- 




INLET 



\ I I 1 I I II I T T 



Machinery 



Fig. 8. 



Arrangement of Piping in Quenching Tank for Hardening 
Forge Tools 



sible and then quench it in water in an ordinary slake tub, until 
the heat has been so far withdrawn that it will " carry water." 
The tool is then polished with a piece of wood covered with 
emery cloth and the temper drawn with the back heat until it 
shows a light copperish color, after which it is cooled off. The 
face is next heated in the same manner as the peen but it is 



174 HEAT-TREATMENT OF STEEL 

cooled in a stream of water rising straight from the bottom of 
the' quenching tub and striking the center of the face, as shown 
in Fig. 8. This insures the center being equally as hard as the 
edges, if not harder, as steam cannot generate and form a cushion 
as it may do where the tool is immersed in the water. When the 
face is fairly cooled to a depth of about i inch, it is polished and 
laid in a hot fire, which in a very short time draws the temper 
on the outer edges to a blue color, leaving the center just as 
hard as possible. When a hammer has been properly hardened 
in the manner described, there is practically no danger of its 
cracking or burring, as the hard center is supported on all sides 
by the softer and tougher metal around the edges. 

The faces of fullers, swages, sets, etc., are hardened in exactly 
the same manner as the face of a hammer, the heads being 
heated slightly above the critical point and quenched to a 
depth of about ^ inch. They are then polished and the temper 
drawn by the back heat until the color has all but disappeared. 
What color is left may be described as light sea-green. Another 
gage for drawing the temper of the heads of tools is to let the 
temper run until the piece is hot enough to freely ignite thin 
scrapings of hickory wood; it is then dipped in water, just enough 
to prevent the temper from running any further, and allowed to 
cool in the air. 

Heat-treatment of Spring Steel. — Experiments have been 
made on spring steel of the following composition: Carbon, i.oi 
per cent; manganese, 0.38 per cent; phosphorus, 0.03 per cent; 
sulphur, 0.03 per cent; silicon, 0.13 per cent. These tests indi- 
cated conclusively that the elastic limit of one-per-cent carbon 
steel can be made to vary from 78,500 pounds per square inch to 
240,000 pounds per square inch by changes in the heat-treatment, 
and that very small changes in the temperature for the drawing 
of the temper are sufficient to affect the elastic limit of the steel. 
The highest elastic limit is obtained by heating to a temperature 
of 1425 degrees F. and quenching in water, and then redrawing 
to 750 degrees F. Better all-round results, however, can be ob- 
tained without drawing the temper, by heating to a temper- 
ature of 1450 degrees F. and quenching in oil. In fact, when 



STEEL CASTINGS 1 75 

quenching in oil, a higher elastic limit is obtained by not draw- 
ing the temper at all. When quenching in water, the steel be- 
comes too brittle for use as springs, unless it is drawn to a temper 
not less than 600 degrees F. 

Heat-treatment of Screw Stock. — The composition of com- 
mon screw stock is usually as follows: carbon, 0.08 to 0.25 per 
cent; manganese, 0.30 to 0.80 per cent; phosphorus, not over 
0.16 per cent; sulphur, 0.05 to 0.15 per cent. It is an unsafe 
steel to use for any parts which require great strength or tough- 
ness. Screws made from it should always be heat-treated and 
should not be used in the annealed condition, especially if made 
from hot-rolled bars. Cold-rolled bars are much stronger than 
the hot-rolled, but the best results from both types are obtained 
after heat-treatment. The heat-treatment generally given to 
this class of steel is to heat it to 1500 degrees and then quench, 
after which it is reheated to from 600 to 1300 degrees F. and 
permitted to cool down slowly. 

The Heat-treatment of Steel Castings. — The Pennsylvania 
Railroad has for some time been experimenting on the effect of 
the special heat- treatment of steel castings, particularly bolsters 
and locomotive frames, and the results of these experiments 
were embodied in a paper presented at a meeting of the American 
Institute of Mining Engineers at New York on February 18, 19 14. 
An abstract of this paper will be of material interest, as it indicates 
what may be done with steel castings when properly treated, 
thereby permitting in railway service greater strength of cast- 
steel parts without any increase in weight or space. The ob- 
scurity formerly surrounding the heat-treatment of steel has 
been for the most part removed by the development of our knowl- 
edge of the critical points of steel, pyrometers, furnace construc- 
tion, and the testing of the finished product. The operations 
of the heat-treatment proper are taken up under the heads of 
1. Heating for quenching; 2. Quenching; 3. Tempering. 

Heating for Quenching. — Heating for quenching is best con- 
ducted slowly, especially in the case of castings of variable thick- 
ness. Cracks may occur either in heating or in cooling, due to 
different temperatures at different points of the casting. The 



176 HEAT-TREATMENT OF STEEL 

castings should be thoroughly soaked at the maximum temper- 
ature (generally 1500 to 1600 degrees F.), one hour being sufficient 
for sections 12 inches in thickness. The minimum temperature 
which will produce the desired hardening effect will, in all cases, 
be found to be the most satisfactory, as the grain coarsens when 
the critical range is exceeded to too great an extent. All tem- 
peratures should be governed by a checked pyrometer with the 
hot junction to the heated object, and with several couples in a 
large furnace to insure a uniform temperature. 

Quenching. — The casting should be transferred as quickly 
as possible from the furnace to the quenching bath, and in the 
case of large castings, such as locomotive frames, this is by no 
means a simple matter. The larger castings are best handled 
by means of cranes and rollers. The quenching agent employed 
is generally water or oil, preferably the former, because of its 
cheapness and cooling effect. With intricate castings it is gen- 
erally best to use oil. With water it is possible to have a large 
tank and a large running stream, serving to maintain a uniform 
temperature. Castings should never be thrown in to rest on 
the bottom of the tank, but should be agitated to prevent the 
formation of a coating of vapor, retarding the quenching effect. 
It is also best, whenever possible, to quench the thicker portions 
first. 

Tempering. — Whenever possible the tempering should be 
done in a bath of some kind, such as lead, barium chloride, a 
barium chloride-salt mixture, or oil. In the case of large castings 
this is manifestly impossible, and great care should be exercised 
in obtaining a uniform temperature in the tempering furnace. 
The use to which the casting is to be put determines the draw- 
ing temperature, railroad work, by reason of the shock and vibra- 
tion of the road, requiring high ductility at the sacrifice of some 
strength; the temperature for this class of work, therefore, is 
about 900 degrees F. 

Process for Hardening Cast Iron. — An improved process 
whereby cast iron in the rough or in the finished state may 
be hardened or tempered, the hardness extending completely 
through articles of comparatively large dimensions, is described 



HARDENING CAST IRON 1 77 

in the following. One of the principal objects of this process 
is to provide a cheap and simple method of rendering iron cast- 
ings so hard that they may be used for many purposes in the 
place of steel, thus reducing the manufacturing cost of a large 
number of articles. The various steps of the process and the 
manner of carrying it out are here described. 

The castings which are to be treated may be completely fin- 
ished as regards machine work before they are hardened. The 
casting is first heated to a " cherry-red " heat; it is then dipped 
in a bath which consists of a practically anhydrous acid of high 
heat-conducting power, preferably sulphuric acid of a specific 
gravity of from 1.8 to 1.9, to which is added a suitable quantity 
of one or more of the heavy metals or their compounds — such, 
for example, as arsenic or the like. The preferable ingredients 
of the bath are sulphuric acid of a specific gravity of approxi- 
mately 1.84 and red arsenic in the proportions of f pound of red 
arsenic crystals to 1 gallon of sulphuric acid. The castings may 
be either suddenly dipped in the afore-mentioned mixture and 
then taken out and cooled in water, or they may be left in the 
bath until cool. We find, however, that dipping the castings 
in the bath and holding them there for a time which varies ac- 
cording to the size of the castings, and then completely cooling 
in water, is quite as satisfactory and produces a material which 
is just as hard as if the castings were allowed to remain in the 
bath until cool, and the former method is preferable if a large 
number of castings are to be hardened, as the bath is thus pre- 
vented from becoming overheated. In preparing the bath when 
sulphuric acid and red arsenic are used, we find that better re- 
sults are obtained when the crystals are added to the sulphuric 
acid and the bath is allowed to stand for about a week before 
using, the reason probably being that the bath becomes more 
saturated with arsenic compound when the dissolved red arsenic 
has been long in the sulphuric acid. 

It is not necessary that the casting be machined completely 
before hardening, as rough-finished castings may be hardened 
equally well. The change which takes place in the metal is in 
the nature of a molecular re-arrangement or re-crystallization 



i 7 8 



HEAT-TREATMENT OF STEEL 



coincident with an increase in the combined carbon at the ex- 
panse of graphitic carbon. 

It is found that the more rapid the cooling of the metal, the 
harder it will become. For this reason the bath must be of 
high heat-conducting power, and this requirement is obtained 
by the use of the ingredients referred to. Furthermore, the 
bath must be practically free from water, as it is found that when 
the acid contains water in any considerable quantity, a steam 
cushion is formed between the acid and the metal which prevents 
their coming in contact, with the result that the cooling is less 
rapid, and, consequently, the iron is not so hard. 




Machinery, N. Y, 



Fig. q. Cooling Tank and Hardening Bath for Cast Iron 

A cylindrical jar made of lead should contain the bath, the 
size varying according to the work to be done. A jar about 
10 inches in diameter by 18 inches deep will be about right 
for ordinary small work. This lead jar should be enclosed in 
an outer vessel through which water is caused to continuously 
circulate in order to keep it cool. It might be further pointed 
out that if it is desired to harden one portion of a casting and 
leave the remaining portion soft, this may be accomplished very 
readily by immersing only the part to be hardened. 

Such a hardening equipment has proved very satisfactory in 
locomotive work for hardening bushings, etc., and many other 
uses could no doubt be found for it. The whole equipment can 
be homemade, consisting as it does simply of a steel tank divided 
into two compartments, and the lead jar for the bath, as shown in 



LOCAL HARDENING 179 

Fig. 9. The water circulates in the first chamber around the lead 
vessel, keeping it cool, and then passes to the second chamber, 
into which the castings are dropped, when taken from the bath, 
to cool off. The water then passes into the sewer or other suit- 
able containing device. There is a screen placed in the second 
compartment to keep the castings from falling to the bottom. 

Local Hardening. — One method of hardening locally is to 
cover the part that is to remain soft with a thin metal shield, 
so that it prevents the surface from being suddenly cooled by 
the direct action of the cooling medium. The steam or vapor 
which forms beneath the cover prevents the cooling medium 
from entering until the work has cooled sufficiently to prevent 
hardening; hence, a rather loose-fitting shield is desirable. The 
shield should be made of sheet iron or steel of about No. 29 
gage (0.014 inch), for ordinary work. It is composed of one or 
more pieces, depending upon the shape of the part, and, when 
several pieces are required, they can be bound together with 
wires or rivets. Of course, the surfaces to be hardened are left 
exposed. The heating should be done in a furnace or open- 
forge fire. A lead bath should not be used, because the hot 
lead beneath the shield will cause an explosion when the part 
is cooled. The quenching bath can be the same as when the 
shield is not used. 

Local hardening is also effected by the application of a com- 
pound called " Enamelite " to the parts which are to remain 
soft. This compound, for tool steel, is in the form of a powder 
which is mixed with hot water to form a paste. It has the 
property of clinging to the steel and liberating hydrogen (the 
greatest known non-conductor) when the heated steel is plunged 
into water. This causes the steel to retain its heat long enough 
to escape the chill, so that it remains soft where the enamelite 
has been applied. 



CHAPTER X 
CASEHARDENING 

General Principles of Casehardening. — Casehardening is 
the process of increasing the carbon content of the surface of 
steel comparatively low in carbon, so that it can be hardened 
by the usual method of being heated to the hardening temper- 
ature and quenched in a cooling medium. The term casehard- 
ening, by itself, implies the hardening of the surface or skin of 
an article, and in order to fully understand the process and its 
object, it is necessary to briefly consider the facts and laws upon 
which it is founded. Carbon has a very great affinity for iron 
and combines with it at all temperatures above a faint red heat. 
Advantage was taken of this fact in the production of steel by 
cementation, an old process which consisted of rolling wrought 
iron into thin strips and then placing these in boxes with some 
material containing a fair proportion of carbon. These boxes 
were then heated to a very high temperature and the carbon was 
gradually absorbed by the iron. 

The process of casehardening is, in fact, only an improvement 
on this old cementation process used in times past for making 
steel from wrought iron. The steel is heated in packing boxes 
in the presence of a carbonaceous material and when the surface 
of the steel has absorbed enough carbon so that it will harden 
the same as high-carbon steel, it can be quenched in oil or water, 
according to the requirements. For many purposes, in machine 
work, articles are required which must have a perfectly hard 
surface and yet be of such internal structure that there is no 
chance of breaking them when in use. In many instances, this 
result can be obtained better by using casehardened mild steel 
than by using high-class crucible steel. For example, in making 
axles, cups, cones, and many similar parts for bicycles, it is ex- 
tremely difficult to obtain perfect hardness combined with great 

1 80 



CASEHARDENING 181 

resistance to torsional, shearing or bending stresses. For such 
purposes, nothing meets these requirements so well as do articles 
which have been casehardened. 

A great improvement has been made in casehardening processes 
during the last few years. The advance was begun with the 
development of the bicycle industry; and the necessity for case- 
hardened parts of the highest quality in automobile manufac- 
ture has caused a still further improvement in this field. 

As an example of what has been accomplished by proper case- 
hardening methods, consider the transmission gearing of an 
automobile. Who would think of throwing in the back-gears 
of a lathe or any other machine tool without first stopping the 
machine? In an automobile, however, this very thing is actu- 
ally done dozens of times a day, by a person who gives little 
thought to what he is really doing. Yet the gears stand up 
under this treatment because of being manufactured of special 
steels developed during recent years and because of being heat- 
treated and casehardened by improved methods. 

There are a number of different questions that must be con- 
sidered in order to obtain good results in casehardening. In 
the first place, the proper kind of steel to be used for various 
purposes must be carefully selected. Another most essential 
thing is that the casehardening furnace must give a uniform heat. 
As oil and gas have to a great extent superseded coal as fuels 
for casehardening furnaces, the changes in furnace construction 
have, of late, been considerable. Another item which must be 
given careful consideration is the box in which the material is 
packed, as well as the carbonaceous material itself used in pack- 
ing the parts to be casehardened. Still another question to be 
dealt with is the method used for hardening the parts after they 
have been carbonized. 

Steel to be used for Casehardened Parts. — As the case- 
hardening process consists in adding carbon to the steel, a 
material must be used which will absorb carbon without neces- 
sitating overheating or burning. The effect of carbon on steel 
is, in general, it may be said, to make it dense, and the denser 
the steel the higher the heat necessary to open the pores through 



182 HEAT-TREATMENT OF STEEL 

which it must absorb the carbon. A low-carbon steel contain- 
ing, say, from 0.15 to 0.20 per cent of carbon is, therefore, most 
suitable for casehardening. It should also be borne in mind 
when selecting the material that the casehardening process does 
not eliminate any of the impurities ordinarily found in iron, 
such as sulphur, phosphorus, etc., and, hence, a material as free 
as possible from these impurities should be selected; besides, 
the material should be perfectly sound and free from mechanical 
faults or weaknesses caused by overheating or improper working 
during the manufacturing processes. 

Both iron and mild steel have been employed as materials for 
casehardening in the past; but this is the steel age, and iron has 
long passed its day. The steel employed should be prepared, 
selected and controlled from the beginning with the object of 
making it suitable for the final requirements. There are many 
points with relation to the selection of the proper steel, its com- 
position and treatment, which can only be gained by long expe- 
rience and a study of the requirements, but, as a general rule, 
the low-carbon steel specified in the preceding paragraph will 
be found suitable for most purposes. 

Hardening Room Equipment. — In Fig. 1 is shown a plan of 
a hardening room especially well equipped for its purpose, and 
containing the requisite furnaces and appliances for casehard- 
ening. This illustration shows the arrangement of the heat- 
treatment equipment of the Boston Gear Works, which is located 
in a separate fire-proof building 26 by 45 feet, consisting of a 
steel framework covered with asbestos-protected metal. The 
equipment comprises two carbonizing furnaces / and K using 
coal as fuel and having heating chambers 24 by 60 inches, and 
also the following oil-burning furnaces: one Westmacott furnace, 
10 by 12 inches, located at B\ one Rockwell furnace, 24 by 36 
inches, located at F; one American Gas Company's furnace, 
20 by 60 inches, located at E; one Stewart crucible furnace, 
located at D. At A is a lead reheating furnace. Besides the 
furnaces the equipment is as follows: H indicates a forge; 7, a 
jib crane; L, a water-quenching tank; M and N, oil-quench- 
ing tanks; and C, a bin for carbonizing materials. The temper- 



CASEHARDENING 



I8 3 



atures are indicated by a Bristol thermo-electric pyrometer, 
located at G. The steel jib crane allows heavy work to be 
easily placed in or removed from the furnaces and cooling tanks. 
The Casehardening Furnaces. — In building or constructing 
a furnace for casehardening, the size of the work to be casehard- 
ened should be the first consideration. It is far better to use 
a small furnace with a small box, whenever possible, than to 
make the furnace larger than is absolutely required. If the 
work varies in size, a number of furnaces of different sizes may 
be used. Small furnaces require less fuel, and small work should 



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Machinery 



Fig. 1. Plan of a Well-equipped Hardening Room 

be placed in small boxes, as otherwise the pieces packed near 
the sides will be overheated before those in the center will reach 
the required temperature. When several furnaces are used, 
these should be made right- and left-hand so that two can be 
placed close together. Thick walls should be used to retain the 
heat. These walls should be supported by substantial concrete 
foundations so that they will retain their position and shape after 
having been repeatedly subjected to the high heat required. 
Large flues should be provided to carry away the smoke and 
gases. 

The furnace should also be so constructed that as much as 
possible of the heat of the combustion gases may be extracted 



1 84 HEAT-TREATMENT OF STEEL 

before they are discharged. The construction should make it 
possible to raise the temperature to a full orange heat (1830 de- 
grees F.) and maintain it at this heat with fair regularity. It 
should also be so constructed that neither the fuel nor the direct 
flame can come in contact with the charge. The flames should 
uniformly impinge on the sides and roof of the muffle in such a 
manner as to raise them to a high temperature, thus heating the 
contents of the muffle by radiation and not by direct heat. A 
furnace designed on this principle not only gives the best results, 
but is also most economical in the matter of fuel. 

The muffle chamber and flues must, of course, be constructed 
of firebrick and the doors should fit closely and also be lined 
with firebrick. The flues and all parts of the furnace should 
be easily accessible. The door should be the full width of the 
oven so that the tiles can be taken out and the flues cleaned. 
It is important that there should be a small peep-hole in the 
door with a cover plate. A hole ij inch in diameter is quite 
large enough. This latter is a most important detail as it pro- 
vides against the need of opening the door in order to judge the 
heat and furnishes the most accurate means of estimating the 
temperature by the eye. The furnace must also be fitted with 
a reliable damper plate or other effectual means of controlling 
the draft. 

Oil-burning Furnaces. — In an oil-heated furnace, a light oil 
having a high heating value and comparatively free from carbon 
deposits should be used. All piping should preferably be placed 
above the furnace so as to be easily accessible. If, however, the 
piping is placed below the ground, it should be arranged in com- 
partments which can be easily reached, if repairs are required. 
A pressure blower should be used and the pipes from the blower 
should be run through the furnace so as to pre-heat the air used. 
If cold air is used directly, it will reduce the heat in the furnace. 
The furnace fronts should be made in several parts to prevent 
cracking, with the door properly balanced and lined. A shelf 
should be provided projecting at the front for holding the boxes 
when they are taken out or put into the furnace. The smoke 
stack should be made of sufficient height to produce a good draft. 



CASEHARDENING 



185 



Burners should be placed both at the front and rear of the 
oven and should be arranged in separate compartments, so that 
the heat will be uniform in the oven. The hot gases will then 
pass over the top of the compartment wall and strike the boxes 



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Fig. 2. Plan and Elevation of Casehardening Furnace 

on the top, after which they pass out through small openings 
in the corner of the furnace. They then take a zigzag course 
under the tiles and pass from there through a flue to the rear of 
the furnace. A large conduit should be provided just below the 
ground which will catch all the soot. This conduit should be 



1 86 



HEAT-TREATMENT OF STEEL 



provided with iron covers which can easily be taken off to remove 
t|ie accumulation of soot. 

The furnace should not be heated too quickly, as this is apt 
to crack the brickwork. The cooling should also be done grad- 
ually. After the work has been taken out and the heat shut 
off for the day, all the dampers should be closed to hold the 
heat. In this way the furnace will cool slowly and cracking or 
bulging out of shape will be prevented. In addition, it will be 
easier to heat the furnace the next morning, as it will have 
retained some of the heat. 



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Machinery 



Fig. 3. Front View of a Casehardening Furnace 

Types of Coal-burning Casehardening Furnaces. — In Fig. 2 

is shown a diagrammatical view of a furnace which may be found 
useful as a guide for the erection of furnaces using solid fuel. 
The upper chamber in this furnace is not necessary for case- 
hardening, but it may be found useful to have such a chamber 
and to employ it for annealing small articles while casehardening 
is being done. This will add only slightly to the amount of fuel 
used. In furnaces not having this upper chamber, work to be 
annealed may be placed in the furnace after the work to be case- 
hardened has been removed and then the furnace may be brought 



CASEHARDENING 



187 




Fig. 4. Section of Casehardening Furnace on line XY, Fig. 5 




Machinery 



Fig. 5. Section of Furnace on line CB y Fig. 4 



i88 



HEAT-TREATMENT OF STEEL 



to the proper heat. The material to be annealed can then 
remain in the furnace until the next morning with the drafts 
closed up and the fire banked. 

The casehardening furnace, shown in detail in Figs. 3 to 6, 
inclusive, is a very good type of hard coal furnace for casehard- 
ening. It can be built from common brick and firebrick and is 
large enough with the dimensions given for ordinary shop work. 




Machinery 



Fig. 6. Section of Furnace on line EF, Fig. 4 

If a larger size of furnace is required, it will be necessary to use 
large tile in place of the firebrick for the bottom of the oven. 
A blast is used in connection with this furnace when starting the 
fire, but very little artificial draft will be required after the box 
containing the work to be hardened is red hot; this, of course, 
depends to some extent upon the draft in the chimney. A 
damper is supplied in a pipe behind the furnace to regulate the 
heat. The following are the principal parts of the furnace: 
A, cast-iron stays; B, f-inch staybolts; C, door frame, f by if- 



CASEHARDENING 



189 



inch iron; D, sheet-iron caps for flues; E, blast pipe, i^-inch 
gas pipe; F, damper; G, damper support; H, cast-iron grate; 
/, grate support; /, blast shut-off; and K, smokestack connec- 
tion. 

Figs. 7 and 8 show the type of casehardening furnace used by 
the Boston Gear Works. This furnace is built of brick and is 
of the same general type as the furnace just described. It is 
a purely reverberatory furnace, the heat being deflected from 
the firebox A over a bridge B down upon the work, the firebox 




Machinery 



Fig. 7. Plan View of Carbonizing Furnace 

A and furnace chamber C being arched over. The gases from 
the furnace chamber C descend through four flues D. These 
flues unite in two ducts E which lead to the chimney flue F. 

Boxes for Casehardening. — Boxes for casehardening should 
not be made larger than is necessary for the class of work being 
handled. They are made both from cast and wrought iron, the 
former being cheaper in first cost, but the latter withstanding 
reheating so many times that they are cheaper in the end. Cast 
iron is also objectionable because it is porous and seems to 
absorb carbon from the carbonizing material. Very good boxes 



190 



HEAT-TREATMENT OF STEEL 



can be made from malleable iron. The precaution of not mak- 
ing them of too large dimensions is quite important, as otherwise 
there is a great risk of the articles in the middle of the charge 
not being carbonized to a sufficient depth on account of the heat 
being low. For such parts as bicycle axles, pedal pins, and the 
like, the box should not be larger than 18 by 12 by 11 inches, 
while for small articles like cups, cones, etc., for bicycles, 12 by 10 
by 8 inches is large enough. The box should have a cover fit- 
ting closely on the inside of the box. 




Machinery 



Fig. 8. Sectional Elevation of Furnace shown in Plan View in Fig. 7 

The boxes should be provided with feet, as shown in Fig. 9, 
so that the heat can circulate all around them. The covers 
should be provided with ribs on the top to prevent excessive 
warping, and the sides of the box should be ribbed so that a fork 
or grapple iron, such as shown in the upper part of Fig. 10, can 
be used for handling the box. The sides of the boxes should 
taper slightly towards the bottom so that the contents can easily 
be dumped out of them. It is also easier to cast the boxes when 
made in this way. A simple truck for handling heavy boxes 
is shown in the lower part of Fig. 10. 

When very large boxes are required, they should, if possible, 



CASEHARDENING 



191 



be provided with a hole through the center, so that the heat can 
reach the contents from the inside as well as from the outside. 
A box of this kind is shown in Fig. 1 1 . For long work, such as 
shafts, tubing, etc., a wrought-iron pipe with a cap on each end 
forms an ideal casehardening box. As a general rule, the boxes 
should not be made too deep in proportion to their other dimen- 
sions, as it makes it more difficult to pack the parts into them if 
made in this way. 




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Fig. 9. Box of Approved Design used for Casehardening 

Packing Materials used in Casehardening. — There is consider- 
able difference of opinion as to the best packing materials to be 
used in casehardening. The carbonizing materials in general 
use are charred leather, powdered bone, cyanide of potassium, 
wood and animal charcoal, prussiate of potash and other materials 
consisting of mixtures of carbonaceous materials and certain 
cyanides and nitrates. For very slight hardening, cyanides are 
often used, but no great depth of case should ever be attempted 



192 HEAT-TREATMENT OF STEEL 

with these. Charred leather gives good results, but poorly 
charred leather and that made from old boots, belting, etc., 
should not be used. A mixture preferred by some to charred 
leather consists of 60 parts of wood charcoal and 40 parts of 
barium carbonate. 

Theoretically, the perfect carbonizer should be a pure form 
of carbon. This being impossible in practice, some claim to 
have obtained the most certain and satisfactory results with good 




Fig. 10. Grapple Iron or Fork used for Handling Casehardening Box, 
and Truck for Handling Heavy Boxes 

charred leather, whereas others advise the use of good fine char- 
coal, pulverized to about the size of kernels of corn. Others, 
again, claim that while there are a great many different kinds 
of hardening materials that give satisfaction, the old-fashioned 
method of using ground bone can always be relied upon to give 
satisfactory results. During the last few years, however, the 
use of bone in various manufactures has increased so that the 
price of ground bone for casehardening purposes is almost pro- 
hibitive. This has caused leather to become more and more 
extensively used for this purpose, the leather being first burned, 



CASEHARDENING 



193 



and then ground and graded. The carbonizer should always be 
perfectly dry and in a granulated or powdered form. 

A mixing bin is of great advantage in connection with the 
handling of casehardening material. Some partly used bone 
and some new is then used to make a mixture suitable for the 
size of the pieces to be hardened. Large pieces require a richer 




Fig. 11. Large Circular Bbx with Hole in Center for the Circulation 
of the Gases of Combustion 

material than smaller ones, because during the higher heat re- 
quired for the larger pieces and the longer application of the 
heat, more carbonizing material will burn away. 

The packing room should, if possible, be separate from the 
room containing the furnaces, so that the packing can be done 
without the discomfort of the heat and dust. Tables on wheels, 
or trucks, provided with shelves of the same height as the shelf 



194 HEAT-TREATMENT OF STEEL 

in front of the furnace and large enough to hold the required 
ntunber of boxes for one furnace, should be provided, so that 
the packed boxes can be easily moved to the furnace and quickly 
placed in it. The work to be hardened should be classified ac- 
cording to its size and the percentage of carbon required, as it 
will take a higher heat for larger work, as well as for pieces which 
are required to absorb a higher percentage of carbon. 

Packing the Boxes. — When packing a box, first put a layer 
of the casehardening material on the bottom, the thickness of 
this layer depending on the size of the pieces to be hardened. 
About i| inch is the minimum depth that should be used. This 
layer should be well pressed down and upon this is placed the 
articles to be hardened. Care must be taken to leave sufficient 
space all around each piece to prevent the parts from touching 
each other or the walls of the box. A space of if inch should 
be sufficient. If the articles are heavy they do not require such 
great care in packing, but if they are thin or long, or have a 
peculiar shape, greater care is required so that the pieces are 
properly supported and cannot sag out of shape. Thin, long 
pieces should, if possible, be placed in an upright position in order 
to prevent trouble from this cause. 

It has been claimed that the precaution of preventing one 
piece from touching another is unnecessary, and that no harm 
is done if parts should be in contact with one another. This, 
however, does not seem a safe method to pursue, because in 
order that the proper case of high-carbon metal may be provided, 
all the surfaces of the parts should be in contact with the carbon- 
aceous material, and if the parts touch one another, it is likely 
that they will be softer at the spots where they are in contact. 
It has also been found from experience that if there is not enough 
of the carbonizing material in the box, the work is liable to have 
soft spots. 

When the first layer of work has been placed in the box, 
another layer of carbonizing material is put over it and well 
pressed down, care being taken not to displace any of the articles 
already packed. Another layer of parts to be hardened is then 
laid on the carbonizer, and this is continued until the box is 



CASEHARDENING 195 

nearly full. About two inches from the top of the box sheet- 
steel strips about jq inch thick should be laid on the last layer 
of carbonizing material and these, in turn, should be covered with 
a layer of about 1 inch or more of powdered charcoal. Then the 
cover is placed on the box and the edges are sealed with fireclay 
or asbestos cement. The clay used for luting around the cover, 
and which is also used for " stopping off " portions to be left 
soft, must be of good quality and free from grease. Clay con- 
taminated with grease in any way will cause irregularity in the 
product obtained. The contents in the box should be packed 
as compactly as possible, because the more solidly the box is 
packed the more complete is the exclusion of air. 

Carbonizing in the Furnace. — The heat required for carbon- 
izing is a great deal higher than that required for ordinary 
hardening. If, for example, the material to be casehardened 
were heated only to 1400 degrees F., which would be sufficient 
for the hardening of ordinary tool steel, the result would be very 
unsatisfactory; in fact, there would be no result at all. Small 
parts must be heated to at least 1575 degrees F., in which case 
sufficient depth of carbonized surface will be obtained in from 
six to eight hours. The time given as correct for casehardening 
should be taken from the time the boxes are heated clear through. 
Ordinarily, however, the proper heat for casehardening is about 
1800 degrees F. for a full orange heat, and^this should be main- 
tained with great regularity throughout the operation. 

The length of time occupied in carbonizing is regulated by the 
depth of casing required and also by the dimensions of the 
article. At the close of the carbonizing period, the box is with- 
drawn from the furnace and placed in a dry place where it is al- 
lowed to become quite cold. It is then opened, the articles taken 
out and brushed over to remove all adhering matter. If the 
box has been properly packed and luted, the articles should be 
quite white, or at least have only a slight film of deep blue 
color. The denser and more inclined to redness the surface 
is, the more imperfect has been the packing and sealing of the 
box. 

If there is any doubt about the length of time required for 



196 , HEAT-TREATMENT OF STEEL 

heating the pieces to obtain a certain depth of case, wire a couple 
of pieces together, allowing the wire to project out of the box. 
These pieces can then be taken out quickly and hardened, and 
in this way it can be ascertained whether the parts have been 
sufficiently carbonized. In casehardening very small work, it is 
advisable to wire the pieces together so that they can be taken 
out of the box at once; otherwise, they would have to be picked 
out with small tongs, as it is impracticable to sift very small 
work in a sieve because the mesh would have to be so fine that 
it would take a long time to do the sifting. 

The boxes should never be put into the furnace under a high 
heat, but should be placed in it when its temperature is from 
800 to 900 degrees F. Then the heat should be slowly brought 
up to from 1500 to 1800 degrees F. In placing the boxes in the 
furnace, great care should be taken that the hot gases have an 
opportunity to circulate all around them. A pyrometer should 
be put in some convenient place and properly wired so that the 
heat in the furnace can be readily ascertained at any time. If 
there is a great deal of night work to be done, a recording pyrom- 
eter should be used as it gives the man in charge a record of the 
heats during the night. 

By the aid of the pyrometer it has been found that when an oil 
or gas furnace is used it is necessary to have an expansion tank 
in order to get a constant air pressure, otherwise the pulsation 
from the blower will affect the heat in the furnace. This ex- 
pansion tank should be situated so that the blower is connected 
directly with one end, while the discharge pipe is connected at 
the opposite end. This will then act as a reservoir, producing 
a constant pressure. When oil is used for the heating, it is 
preferable to pump it from the storage tank in the ground to a 
stand pipe, which will insure a constant flow of the oil. The 
intermittent action of the pump, should the oil be used directly 
as it comes from it, is objectionable. There is also another 
advantage, in case the pump should have to be shut down on 
account of break-down. In that case, the furnaces could still 
continue to operate, as the stand pipe should hold a supply of 
oil sufficient for several hours. At night and on holidays the oil 



CASEHARDENING 197 

should be drained back into the storage tank in order to mini- 
mize the danger incident to its use. 

The supply pipe for the air should come from the outside and 
should be so arranged that the air passes through a fine wire 
netting, so as to prevent foreign substances from entering the 
blower. 

On the outside of each furnace a card should be placed telling 
the kind of work that is in the furnace, when the work was put 
in, the heat required for it, and when it is to be removed. These 
cards can be kept as a record which will be of value when com- 
parison is made with the depth of case obtained under any specific 
conditions. 

Experiments to Ascertain the Proper Carbonizing Temper- 
ature. — Although the proper temperature for casehardening 
is about 1830 degrees F., this temperature may be modified to 
suit the purpose in view. The absorption of the carbon com- 
mences when the steel reaches a low cherry-red heat (1300 de- 
grees F.) ; it begins, of course, at the outer surface and gradually 
spreads until the whole of the steel is carbonized. The length 
of time this requires depends upon the thickness of the metal 
being treated. The percentage of carbon absorbed is governed 
by the temperature, and although the increase of carbon is not 
in uniform proportion to the rising temperature throughout, it 
is perhaps sufficient for our present purpose to note that at 1300 
degrees F., iron, if completely saturated, can contain no more 
than about 0.50 per cent carbon; at 1650 degrees F., about 1.5 
per cent carbon; and at 2000 degrees F., about 2.5 per cent. 
These results, however, are only obtainable when the whole 
section of the iron has received all the carbon it is capable of 
absorbing at the given temperature, and is therefore in a state 
of equilibrium. From this it will be seen that if the process is 
stopped before the action is complete, the central parts of the 
iron must contain less carbon than the outside, and upon this 
fact the process of casehardening is founded. 

If we take two pieces of f inch diameter round mild steel, and 
heat one of them with a carbonizer at a cherry-red heat, and 
the other at a bright orange heat, for six hours, the first will 



198 HEAT-TREATMENT OF STEEL 

be cased to a depth of about -^ inch, and the other to a depth 
of nearly ^V inch, while the amount of carbon taken up will be 
about 0.50 and 0.80 per cent, respectively; so that, so far as 
regards the hardness of the skin, the piece carbonized at the 
higher temperature gives the best result. From this we learn 
that a temperature of 1830 degrees F. will give us sufficient 
hardness of case. 

We have next to find which temperature has the least harmful 
effect on the mild steel core, and this can best be found by 
heating pieces of the mild steel at varying temperatures at and 
above the selected one for the same length of time, using lime 
or other inert substance in the pot instead of a carbonizing 
material, and afterward reheating and quenching in water. 
Suppose, for example, we take three pieces, heating at 1830, 
2370 and 2730 degrees F., or full orange, white and bright white 
respectively. We shall find that those at 2370 and 2730 degrees 
break very short and have lost nearly all their original tenacity, 
while that at 1830 degrees appears tougher and altogether 
stronger than before. 

Having arrived at a knowledge of the right temperature, it 
remains now to inquire as to the length of time requisite to yield 
a sufficient depth of case. At a full orange heat a bracket cup 
of ordinary dimensions should in two hours be hardened -^ inch 
deep, and a bracket axle of \\ inch diameter in 6 hours would 
have a case -^ inch deep. From this it will be seen that the speed 
of penetration is not in exact proportion to the time of heating. 

Variations in Casehardening Methods. — The correct way in 
which to caseharden is first to carbonize the material and then to 
allow the boxes to cool down with the work in them, after which 
they are reheated and hardened in water. The reheating refines 
the grain of the steel and prevents the formation of a distinct 
line between the outer hardened case and the soft core. If there 
is a distinct line between these two sections, the case is liable to 
flake off when the hardened part is subjected to severe stresses. 

A still more refined method of casehardening is to repack the 
work, after it has been carbonized, in old bone, and after heating 
for two or three hours take it out and dip the pieces in the hard- 



CASEHARDENING 



199 



ening tank directly as they come from the boxes. This will 
produce a very fine grain and in many cases prevent warping. 
If the work is large and it is required to toughen the inner core, 
it should be reheated to a higher heat than otherwise; then, 
after dipping, reheat again to 1500 or 1600 degrees F. according 
to the size of the work, and redip. 

However, if the work to be hardened consists of bolts, nuts, 
screws, etc., it is satisfactory to dump them into water directly 
from the furnaces, without any reheating. A regular iron wheel- 
barrow with two pieces of flat iron placed across it lengthwise 
should be provided. On top of these bars is placed a sieve 
made from J -inch wire with |-inch mesh, about 18 inches square 
by 6 inches deep. This sieve should have a handle 6 feet long 



















Machinery 



Fig. 12. Mandrel used when Hardening Collars and Similar Parts 
on the Outside only 



and f inch in diameter. The boxes are emptied into this sieve, 
and after sifting, the heated material is dumped into a tank 
of cold water which should be of sufficient size to prevent the 
water from heating too quickly. Care should also be taken in 
emptying the contents of the boxes into the water that they 
are not all dumped in one place, but scattered about in the tank. 
A constant flow of water should be available while the work is 
being hardened. The work should under no circumstances be 
removed from the furnace until the heat has been lowered, as 
the steel should be treated as tool steel after it is carbonized, 
and it would be injurious to the steel to harden it at the high 
carbonizing heat. 

Gears and other parts which should be tough, but not glass 
hard, should preferably be hardened in an oil bath. There is 



200 



HEAT-TREATMENT OF STEEL 



then less liability of warping the work, and the hardened prod- 
uct will stand shocks and severe stresses without breakage. 
Cotton-seed oil is the best hardening medium to be used in this 
case. 

Reheating and Hardening. — After the work has been prop- 
erly carbonized, the next operation in the case of all parts, except 
those mentioned as exceptions above, is, therefore, to reheat. 
This may be done either as already explained, or it may be done 
in a regular muffle gas furnace in which the work can be put 
in rows on the tile. In this way the work can be heated very 
slowly, a new piece being put into the furnace to take the place 
of each piece as it is removed. Collars, etc., which are required 
to be hardened on the outside, but ought to be left soft on the 



CUP-SHAPED 
HOLDER 




Fig. 13. Holder for Parts which are to be Hardened Locally 

inside, should be hardened on a mandrel, such as shown in 
Fig. 12, the diameter of the mandrel being from 0.001 to 0.003 
inch smaller than the hole in the piece to be hardened. If the 
inside of the piece only needs to be hardened and the outside 
should be left soft, a cup-shaped holder, such as shown by the 
dash-dotted lines in Fig. 13, may be used. In this case the 
work will harden at B while it is left soft at A and C. 

Reheating Furnaces. — It is important that a suitable fur- 
nace should be employed for the purpose of reheating casehard- 
ened articles before quenching. It is not advisable to reheat 
in the same furnace as is used for carbonizing unless the furnace 
is run specially for this purpose and at a lower heat. Ordinarily, 
a small gas or oil-heated furnace can be used to advantage for 
reheating. A properly constructed gas or oil-heated furnace 



CASEHARDENING 201 

can be regulated with great exactness and this is very important 
in all hardening. 

Quenching Baths. — It is generally advisable when quench- 
ing casehardened parts to use brine or salt water rather than 
pure water, because the latter does not chill the parts quickly 
enough. The hardening tank should be about 30 inches in 
diameter and 36 inches deep and have a constant flow of water 
from a pipe in the center about 6 inches below the surface. 
When extreme hardness of case is not required, pure water will, 
of course, give satisfactory results, and where the hardness of 
the surface can be still further sacrificed for a tenacious struc- 
ture in the material, oil quenching baths are frequently used. 

American Society for Testing Materials Methods of Case- 
hardening. — A committee of the American Society for Test- 
ing Materials recommended the following practice for case- 
hardening carbon-steel parts. Four different conditions were 
considered, varying from the heat-treatment that would give 
the hardest surface and the least strength, to that which would 
give the greatest strength with the least hardness of surface. 
When a hard case is the only requirement and lack of toughness 
or even brittleness is unimportant, the articles may be quenched 
by emptying the contents of the casehardening boxes directly 
into cold water or oil. In this way both the core and the case 
are coarsely crystallized and the strength is reduced. If the 
articles are allowed to cool to a temperature slightly exceeding 
the critical range of the casehardening, usually from 800 to 
825 degrees C. (1472 to 1517 degrees F.), and then quenched, 
the core and case still remain crystalline, but the danger of dis- 
tortion or cracking in the quenching bath is reduced and the 
strength is somewhat increased. The next recommended method 
is to increase the toughness and strength of the article and refine 
the case. The articles are allowed to cool slowly in the carbon- 
izing pot to a temperature of about 650 degrees C. (1200 de- 
grees F.), are then reheated to a temperature slightly exceeding 
the lower critical point of the case, which usually is from 775 to 
825 degrees C. (1427 to 1517 degrees F.), and are then quenched 
in water or oil. They should be removed from the quenching 



202 HEAT-TREATMENT OF STEEL 

bath before their temperature has fallen below ioo degrees C. 
(/212 degrees F.). By allowing them to cool slowly to a tempera- 
ture of about 650 degrees C. (1200 degrees F.) and then reheat- 
ing to a temperature of about 900 to 950 degrees C. (1652 to 1740 
degrees F.), followed by quenching in oil, from which they are re- 
moved before they have dropped below a temperature of 100 de- 
grees C. (212 degrees F.), then reheating to about 800 degrees C. 
{1472 degrees F.) and again quenching in water or oil, both the 
case and the core will be thoroughly refined and their toughness 
greatly increased. In order to reduce the hardening stress 
created by quenching, the objects, as a final treatment, may be 
tempered by reheating them to a temperature not exceeding 
200 degrees C. (392 degrees F.). 

Results of Hardening without Reheating. — That part of the 
process where a most important improvement has been made 
in recent years is in the final hardening by quenching in a suitable 
bath. It formerly was customary at the end of the carbonizing 
period to open the pot and fling the contents headlong into a 
tank of cold water. Here and there some of the more careful 
workers took each article separately, but direct from the pot, 
and plunged it into water. These latter obtained better results, 
but even they had a great deal of trouble in the way of break- 
ages and want of regular hardness. Finding that axles taken 
singly from the pot and quenched were better than those quenched 
in bulk, and that if allowed to cool down to cherry-red they were 
better still, an application of the old rule to harden on a rising 
heat led to the now established principle of allowing the pot and 
its contents to become quite cold, afterward reheating to cherry- 
red and quenching with water. By this means a case of great 
hardness with a very tough core is obtained — that is, of course, 
provided a suitable steel is employed. 

To understand the reason of this improved method of working 
it must be remembered that the exterior of the steel is now of 
about 0.80 per cent carbon, and that steel of all kinds raised 
to and maintained at the high temperature employed for case- 
hardening will, unless subjected to mechanical work, show evi- 
dence of overheating, being very brittle and liable to easy fracture; 



CASEHARDENING 203 

and though quenched in water, and consequently hardened, the 
metal has little or no cohesion and readily wears away. Steel so 
hardened breaks with a very coarse crystalline fracture, in which 
the limits of the case are badly denned. It is known that when 
steel is gradually heated there is a certain point at which a great 
molecular change takes place, and that perfect hardness can only 
be obtained by quenching at this critical point. 

If quenching takes place below the critical temperature, the 
steel is not sufficiently hard; if above, though full hardness may 
be obtained, strength and tenacity are lost in part or completely, 
according as the critical temperature is exceeded by much or 
by little. This critical point lies between 1380 and 1470 de- 
grees F., or cherry-red color heat. It may be asked why it is 
not sufficient, when taking the article out of the pot, to allow 
it to cool down to cherry-red and then quench it. To this the 
answer is that the high temperature has already created a 
coarsely crystalline condition in the steel, and that until it has 
become quite cold and has again been heated up to the critical 
temperature, a suitable molecular condition cannot be obtained. 
When steel is cooled suddenly, it bears in its structure a con- 
dition representative of the highest heat to which it was last 
subjected. 

Casehardening in Cyanide. — The cyanide is melted in a 
cast-iron pot in a furnace and then the work to be casehardened 
is entirely immersed in the cyanide, which is heated to a bright 
cherry red. The work should be suspended by fine iron wires. 
Fifteen or twenty minutes after the work has been thoroughly 
heated through, it can be removed, and a casing of suitable 
depth for ordinary purposes is insured. The length of time of 
immersion will simply add to the depth of the casing, but thirty 
minutes of heating will give a very deep casing. The work can 
be dipped in clear cold water immediately after having been 
removed from the cyanide bath, or it may be permitted to cool, 
be reheated and hardened in the approved manner for case- 
hardening. When small pieces are to be heated in cyanide, it is 
best to use wire baskets. These must be st> made that the liquid 
has free access to all the surfaces of the finished pieces. 



204 HEAT-TREATMENT OF STEEL 

Local Hardening. — In many cases it is essential that the 
piece of work be hardened at a certain place and that other 
parts be left soft. There are three ways in which this can be 
accomplished: First, by copper-plating and enameling; second, 
by covering the part which is not to be hardened with fireclay; 
and third, by using a bushing or collar to cover the part to be 
left soft. 

In the first case the article should be painted with enamel 
where it is to be hardened, the enamel being baked after having 
been applied. The remainder of the piece that is to be left soft 
is copper-plated. In the second case, if the article to be hard- 
ened has a recess, such as a hole, slot, etc., this may be filled 
with clay. The third method is used when a shaft, for example, 
is only to be left soft for a short distance. A collar is then 
placed on the shaft, and this provides the easiest and least ex- 
pensive means for accomplishing the purpose. 

In the case where enamel and copper-plating is used, the enamel 
will burn away and allow the surface covered by it to absorb 
carbon and, hence, to be hardened, whereas the copper will 
stand a very high heat and prevent hardening of those portions 
that are covered by it. If the copper is burned off, it is an in- 
dication that the work has been overheated. The clay prevents 
the hardening of a portion of the work in the same way as does 
the copper. It is also of advantage when dipping the work, as 
it prevents the formation of steam pockets which are apt to 
warp or distort the piece. When a sleeve or collar is used, this 
should be made about one-half inch longer than the part which 
is to be left soft, so as to prevent carbonization near the ends 
of the collar. 

Casehardening Alloy Steels. — When nickel steels are heat- 
treated by casehardening, nickel seems to retard the process 
somewhat and the hardness of the " case " is somewhat lower 
than that obtainable in ordinary carbon steels. On the other 
hand, nickel tends to oppose the crystallization of the steel at 
high temperatures and to eliminate the consequent brittleness. 
With a 2 per cent nickel steel, the following temperatures are 
recommended : The steel should first be quenched from a temper- 



CASEHARDENING 205 

ature of 1830 degrees F. It is then given a second heating to 
1380 degrees F., and is again quenched, after cooling to about 
1290 degrees F. A single quenching from 1290 degrees F. gives 
the greatest hardness in the case, but not the greatest tenacity 
in the core. Quenching from 1380 degrees F. gives a somewhat 
higher tenacity but a slightly lower hardness in the case. A 6 
per cent nickel steel should be quenched first from 1560 degrees 
F., and after reheating, from 1245 degrees F. Since this high 
nickel percentage almost completely prevents the brittleness of 
the core, one quenching from about 1290 degrees F. is, in most 
cases, sufficient. Steels with from 1 to 1.2 per cent chromium 
are sometimes used when an especially hard case is required. 
This element aids the crystallization of the core and the double 
quenching is necessary. Chrome-nickel steels with a low chro- 
mium content require about the same heat-treatment as pure 
nickel steels. A mixture of 60 parts wood charcoal and 40 parts 
of barium carbonate is recommended for carbonizing. 

Casehardening Practice at the Juniata Shops of the Penn- 
sylvania R.R. — The information in the following relates par- 
ticularly to the method of packing the work, and to the use 
of test pieces to determine the quality and depth of the harden- 
ing. The following formula is used as a packing mixture: n 
pounds prussiate of potash, 30 pounds sal soda, 20 pounds coarse 
salt, 6 bushels powdered charcoal (hickory preferred). 

The whole is mixed thoroughly, using about 30 quarts of 
water in the mixing; the above quantity is sufficient to harden 
three boxes of material containing the following parts: 2 links, 
2 fink blocks, 2 link-block pins, 2 valve-rod pins, 4 knuckle-joint 
pins, and 24 gibs for spring rigging. 

The box required to hold these parts measures 40 inches long, 
16 inches wide, and 12 inches deep; the time required to harden 
them properly is fourteen hours. Smaller parts, like link die 
plates, eccentric-rod jaw pins, and nuts below 1 inch, are usually 
packed in smaller boxes, or pipe 8 inches in diameter, which 
require between four and five hours heating. Link motion 
bushings and similar light parts are also packed in small boxes 
or pipe and require from two and a half to three hours. 



206 HEAT-TREATMENT OF STEEL 

Packing the Material. — The following method is pursued in 
packing the material: The bottom of the box is covered to a 
depth of 2 inches with the compound; the parts to be hardened 
are placed solidly, so that the compound is in contact with 
the bottom surface of the work; care is taken, however, that the 
work does not touch the sides of the box or other pieces in the 
box. After the first layer of the material is placed, it is covered 
on all sides and on the top with the compound, and is solidly 
packed with a suitable implement — a bolt with a large head 
will do. After the first layer is packed the same process is re- 
peated, being careful to have sufficient compound between the 
two layers to prevent contact. There should not be less than 
2 inches of packing material on top of the last layer. The lid 
which fits inside of the box is then thoroughly sealed with a lut- 
ing of fireclay. 

When in the furnace, the box rests on rollers to allow the 
flames to pass under it. The furnace is kept at a bright red heat, 
but not hot enough to scale or blister the work; when the material 
has soaked in the fire a sufficient length of time, the box is with- 
drawn to a trestle which is flush with the floor of the furnace, 
and stands parallel with and close to the water tank. The lid 
is then removed from the box and, if links are being hardened, 
the link is turned on edge and a bar passed through the slot in 
the link and lifted by two men, being plunged into the water 
endwise; the bar is then withdrawn and the link is allowed to 
remain in the tank until cold. 

The results required in this case are obtained by emptying 
the contents of the box into the water at once. The tank used 
has a line of i^-inch iron pipe connected with the service pipe, 
running around the four sides and close to the bottom, with J- 
inch holes drilled about i| inch apart. This supplies cold water 
to the work and drives the hot water to the top, where it is carried 
to the sewer by means of an overflow pipe. 

The Use of Test Pieces. — With each box of material to be 
hardened a test piece is used. In the case of links, the test piece 
is i J inch thick by 3 inches wide, and 12 inches long; this test 
piece is stenciled with figures giving the class of engine, the con- 



CASEHARDENING 207 

struction number and the date of hardening. After the links 
are taken out, the test piece is broken under hydraulic pressure 
and examined for depth of hardening, after which it is also 
subjected to a file test. Smaller parts are similarly tested. 
These test pieces are kept for two years or more for reference. 
It is possible to put from yV i ncn to -^ inch depth of case on 
the link work in fourteen hours' time, and jq inch on bush- 
ings and other small parts in from two and a half to three 
hours' time. All parts to be casehardened must be thoroughly 
cleaned. 

Degree and Depth of Hardened Surface. — The percentage 
of carbon contained in the casehardened surface should vary 
according to requirements. A high-carbon case containing 1.1 
per cent carbon gives a very hard wearing surface suitable for 
work that must withstand a fairly constant pressure, as shafts 
running in bearings, etc., but for parts which must withstand 
repeated shocks, this amount of carbon would render them too 
brittle, and in such cases it is advisable not to exceed 0.90 to 1 
per cent carbon. For most purposes, 0.90 per cent carbon is 
preferable. Recent investigations indicate that the percentage of 
carbon in the hardened crust varies with the depth of the latter; 
the deeper the penetration, the higher the carbon content. 
Crusts about 0.050 inch deep usually have from 0.85 to 0.90 per 
cent carbon on the surface. In many instances, a penetration 
of 0.040 inch is sufficient, but if the work is to be ground after 
casehardening, it is advisable to carbonize to a depth of about 
Yq inch. Too deep a carbonized case makes the work more 
brittle, partly because of the prolonged exposure to a high tem- 
perature and partly on account of the increase in the hardened 
section and the decrease in the softer and more ductile core; 
hence, parts to withstand bending stresses, like gear teeth, should 
not be carbonized too deeply. The penetration of the carbon 
increases with the temperature and with the time of exposure, 
but not in direct proportion to these two factors. Carboniza- 
tion takes place rapidly until the crust is saturated with carbon, 
when there is a sudden diminution in the rate of carbonization, 
which varies according to the temperature. 



208 HEAT-TREATMENT OF STEEL 

To Clean Work after Casehardening. — To clean work, espe- 
cially if knurled, where dirt is likely to stick into crevices after 
casehardening, wash it in caustic soda (i part soda to 10 parts 
water). In making this solution, the soda should be put into 
hot water gradually, and the mixture stirred until the soda is 
thoroughly dissolved. A still more effective method of cleaning 
is to dip the work into a mixture of i part sulphuric acid and 2 
parts water. Leave the pieces in this mixture about three min- 
utes; then wash them off immediately in a soda solution. 

Straightening the Work after Hardening. — On account of 
the manner in which steel is rolled, drawn or forged, the density 
varies in different parts of the steel, and no matter whether the 
material is heat-treated or not, it will warp more or less when 
hardened. It is, therefore, necessary to provide apparatus for 
straightening the work. In straightening, it is necessary to bend 
the work about twice as much as would be required to merely 
keep it straight while the pressure is applied, as, on account of 
its elasticity, it will have a tendency to work back to its original 
form. Small rollers and shafts can best be straightened in a vise 
by having a three-point contact on the jaws. For large diam- 
eters a special straightener will be required. A surface plate 
placed to the height of a man's eye, and at a slight angle towards 
the light, provides the easiest means for testing work of this 
character while being straightened. 

When there is a large quantity of rings to be straightened or 
trued up, a surface plate can be readily rigged up in the follow- 
ing manner: A solid strap is provided on one side and a com- 
pound lever on the other, adjustable to any place along the plate 
by means of a slot in the latter. By a slight movement of the 
lever the ring can be trued up. An indicator should be placed 
at the front of the plate so that the operator can try a ring to 
see at which points the ring is out, and also the amount neces- 
sary for making it round. In straightening washers or flat 
pieces of any kind, the hydraulic press provides the best possible 
means. It might be well to mention that washers or flat pieces 
should be ground by taking a small amount off each side alter- 
nately, as, otherwise, they will return to their original warped 



CASEHARDENING 



209 



shape. Another precaution, relating to the grinding of cylin- 
drical surfaces, is to use a copious supply of water, as otherwise 
the heat of the grinding operation will draw the surface, pro- 
ducing soft spots. These will appear to have been caused by 
improper casehardening, but as a matter of fact, they are wholly 
produced during the grinding operation. 

Casehardening for Colors. — When hardening for colors, a 
furnace like the one shown in Figs. 3 to 6 is necessary. A very 
satisfactory method of coloring, which at the same time hardens 
deep enough for the class of work which is to be colored, such 
as wrenches, crank levers for automobiles, nuts, etc., is as follows: 
Mix 10 parts charred bone, 6 parts wood charcoal, 4 parts charred 




Machinery 



Fig. 14. Hardening Tank Arranged for Mottling and Coloring 



leather and 1 part powdered cyanide. The charred bone may 
be obtained by placing a few boxes of raw bone in the furnace 
on Saturday night (if the furnace is not run over Sunday). If 
much fire is in the firebox it should be drawn, as the heat in the 
furnace will be sufficient to char the bone to a dark brown. The 
charred leather may be obtained in the same way. The leather 
should be black, crisp and well pulverized, and the four ingre- 
dients should be well mixed together. The object in charring 
the bone and leather is to remove all grease. The parts to be 
colored must be well polished and they should not be handled 
with greasy hands. These rules must be observed if a nice class 
of work is desired. 

If the colors obtained are too gaudy, the cyanide may be left 
out, and if there is still too much color, leave out the charcoal. 



2IO HEAT-TREATMENT OF STEEL 

When packing parts to be colored and hardened, they should be 
packed in a common gas pipe, for the reason that when dumping 
into the water the parts must not be exposed to the air, and a 
pipe is much easier to handle than any other shape. The open 
end can be brought down close to the top of the water before the 
parts are liable to come out, but not so with a box, for just 
as soon as a box is tipped a little the parts begin to fall out, and 
become exposed to the air. 

In heating this class of work, heat to a dark cherry-red and keep 
at that heat for about four or five hours; if heated too hot, no 
colors will appear. To harden and color the work when dumped, 
a tank must be arranged as shown in Fig. 14. A compressed-air 
pipe A must be connected with the water pipe B, and a large 
cap C should be drilled full of J-inch holes on top and around 
the sides. An overflow D should also be supplied. Fill the 
tank with water, then turn on air enough to fill the tank with 
lively bubbles and dump the work in the center, at C When the 
work is all dumped, pull up the sieve which should be in the tank 
for the work to fall into, pick the work out and place it in pails 
of boiling water drawn from the boiler; let it remain for five 
minutes and then remove it to a box of dry sawdust for half 
an hour; remove it from here and dust it off and give it, finally, 
a coating of oil. 



CHAPTER XI 
NEW CASEHARDENING METHODS 

The Development of Casehardening by Carbonaceous Gas. — 

During the past decade there has been a great deal of investi- 
gation relating to the conditions under which the various 
carbonaceous gases may be used in place of the familiar solid car- 
bonizing materials. The old well-known casehardening process 
described in the preceding chapter was the only one known for 
many centuries. It was used without a question of its superi- 
ority until the manufacture of armor plate became such a large 
industry that efforts were made to find a better, or cheaper, way 
of causing the carbon to penetrate this plate. 

The first method employed was to place an armor plate in a 
pit and cover it with a layer of charcoal, and then lower another 
plate onto it. The cover was then put on the pit and the plates 
heated to (or baked in) a temperature that was sufficient to cause 
them to absorb the carbon from the charcoal. Gas was gener- 
ally used for the fuel, owing to the ease of controlling the heat. 
The next method tried was to send a current of carbonaceous 
gas between the two plates, in place of the charcoal. This 
caused the carbon to "soak in" in less time and was found 
more economical. Later, electricity was used for heating the 
plates, and with the carbonaceous gas and electricity, the carbon 
penetration was found to be more uniform over the entire surface 
of the plate. 

The results obtained from the action of carbonaceous gas on 
armor plate have been such that a muffle carbonizing furnace 
has been built and placed upon the market. This machine, 
illustrated and described in detail in succeeding pages holds the 
work in a revolving retort, through which is sent a current of 
carbonaceous gas. This retort serves as a muffle that is sur- 
rounded with the flames of the heating gases. With this furnace, 

211 



212 HEAT-TREATMENT OF STEEL 

small pieces can be carbonized in much less time than formerly, 
and at about one-half the cost as compared with packing in iron 
boxes and then baking in an oven furnace. All of the labor of 
packing materials is done away with; carbon will penetrate the 
metal in less time and more evenly; its depth and percentage 
can be controlled more easily; and the work can be heated to 
the carbonizing temperature more quickly and maintained there 
more easily. A steady flow of carbonaceous gas can be kept 
passing through the retort, and thus any depth of carbon can 
be obtained without repacking the work. 

Comparison between Old and New Methods. — When con- 
siderable depth of carbon is required, this is impossible with the 
old method of packing with bone and charcoal in an iron box 
and sealing on the cover. This is due to the fact that only a 
certain amount of carbon is present, and the longer the work is 
baked, the more there will be in the steel and the less in the 
charcoal. When an equilibrium is established, no more carbon 
will penetrate the metal, and to obtain a greater depth, the work 
must be packed in fresh carbonaceous material and the heating 
repeated. 

With the gas process, however, the percentage of carbon in 
the gas surrounding the work can be maintained at a permanent 
figure until the carbon has penetrated to the center of the metal, 
the percentage of carbon possible to impart to the steel being 
far above that which is used for any kind of commercial work. 
Some of the gases that have been experimented with are methane, 
ethylene, illuminating gas, carbon monoxide, carbon dioxide, 
and gases that are made from liquids like petroleum, naphtha 
and gasoline. Most of these gases have been used in com- 
bination with ammonia, in order to ascertain to what extent 
this would aid in the penetration of the carbon. 

The Carbonaceous Gas. — From the numerous experiments 
that have been conducted, it has been found that carbon monox- 
ide is far superior to any of the solid carbonaceous materials 
in the specific, direct carbonizing effect it has upon steel. It is 
also better than all other gaseous materials in this respect. Car- 
bonizing materials that do not contain nitrogen cost only from 



CASEHARDENING 213 

one-tenth to one-twentieth of the nitrogeneous materials. It 
has been found, however, that nitrogen acts as a carrier for the 
carbon, and when it is not present, carbonaceous materials 
have a very weak carbonizing effect; some investigations have 
shown that the effect is absolutely nil without the intervention 
of gaseous carbon compounds. When solid carbonaceous mate- 
rials are used, the specific effect of the nitrogen is very weak, and 
it is only when these contain a high percentage of the cyanogen 
compounds that they have any marked carbonizing effect. 

While carbon monoxide is capable of rapid penetration, it 
has an oxidizing effect on steel, and is liable to form a scale that 
will spoil small work which cannot afterwards be ground. This 
oxidizing effect is more pronounced in chromium and manganese 
steels. When carbon monoxide alone is used for the carbonizing 
medium, there is a distinct demarkation between the carbonized 
zone and the core of the metal. This is also a detrimental 
feature, in that when the piece is hardened, it has a tendency 
to crack at this demarkation, causing the outer shell to peel off. 

The Giolitti Process. — To overcome these bad effects of car- 
bon monoxide, a new process has been developed by Dr. F. 
Giolitti, Genoa, Italy. In this process the work is packed with 
wood charcoal in a cylinder, and when heated to the carbonizing 
temperature, a current of carbon dioxide is injected into the 
cylinder. It was demonstrated that when a slow current of 
carbon dioxide traversed a mass of wood charcoal, the carbonizing 
gas was supplied with great rapidity and without any excess of 
carbon monoxide. Thus, an eqiiilibrium with free carbon was 
established at the carbonizing temperature. The exhaust gas 
contained less than three per cent of carbon dioxide, it being 
almost entirely carbon monoxide, and its volume being about 
double that of the carbon dioxide which was introduced into the 
apparatus. Some results that were obtained with carbon monox- 
ide alone, and in combination with charcoal, are shown in Table I. 

With the use of this new process, a more rapid penetration 
can be obtained than with any of the solid or gaseous materials, 
except pure carbon monoxide. The carbon is evenly distributed 
in the carbonized zone, and the peeling of the outer shell, when 



214 



HEAT-TREATMENT OF STEEL 



hardened or tempered, is reduced to a minimum. Any desired 
depth of penetration can be obtained without renewing the car- 
bonizing material, and there is absolute security against the 
introduction of any foreign substance. Variations in the per- 
centage and depth of the carbon can be obtained by diluting 
the carbon monoxide in nitrogen; by limiting the contact of 
the solids with the metal; and by varying the temperature dur- 
ing the carbonizing operations. Expansion or contraction and 
warping of the pieces being carbonized has also been reduced to 
a minimum. In fact, there are so many features that make 
it superior to the old method of packing and sealing the work to 

Table I. Results of Carbonizing Steel with Carbon Monoxide for 
Ten Hours at 2000 Degrees F. 



Depth from 
Surface at which 

Sample was 
Analyzed, Inches 


Percentage of Carbon 


Carbon Steel 


Nickel-chromium Steel 


CO Gas Alone 


C0 2 and Charcoal 


CO Gas Alone 


C0 2 and Charcoal 


H4 

He 

Me 


O.70 
O.67 
0.53 
0-39 


1. 17 

O.81 
0.34 


O.86 
O.87 
0.7S 
o-59 


1. 16 
O.81 
O.SO 



be carbonized in iron boxes, that it is safe to say that the new 
process is incomparably better. 

After much experimenting, Dr. Giolitti decided that the double 
muffle furnace, shown in Fig. 1, was the most efficient and eco- 
nomical for carbonizing small or medium size pieces of varying 
shapes. By providing two muffles, one could be filled with 
work and kept at the carbonizing temperature, while the other 
was being emptied and refilled. If the amount of work would 
warrant, many more muffles could be used in the same furnace. 
The muffle to the right is shown by a sectional view through the 
center of the furnace, but the retort that holds the work is not 
sectioned, while the muffle to the left is shown by a sectional 
view on the center line, thus revealing the interior. 

Description of the Giolitti Furnace. — Cylindrical muffles A 
are made of some refractory material and are built into the 



CASEHARDENING 



215 



pftjljlp 



^w^'^mmd 




Machinery 



Fig. 1. Muffle Furnace for Carbonizing Steel with Charcoal and 
Carbonaceous Gas 



2l6 HEAT-TREATMENT OF STEEL 

brick-work of the furnace. Surrounding them are passages B, in 
which the combustion of the heating gases takes place. These 
passages are lined with firebrick or fireclay and the furnace is 
provided with regenerators, so that the fuel gas, which is furnished 
by producers, can be used in the most economical manner. By 
this arrangement and the valves that are supplied, it is possible 
to maintain the work at a uniform predetermined temperature. 
The work-holding retort C is made of seamless steel tubing and 
sets into flange D, which latter is attached to a frame that fits 
into the brick-work underneath the heating chamber. Flange 
E, which is made U-shaped to hold cover R, supports retort C 
at the top of the furnace. Thus, it is only the work of a few 
minutes to take out retort C and replace it with a new one, when 
it has become warped out of shape or burnt through. 

Inside of retort C is located a hollow cast-steel cylinder F, 
and inside of this is located a device that evenly distributes the 
carbonizing gas around the work in the retort, by sending it 
through cover plate G, which is filled with holes. When all of 
the carbon is taken from the gas, it is allowed to escape through 
vent H. The work to be carbonized is stacked up on cover 
plate G, which rests on casting F, and this, in turn, rests on the 
same flange D that supports the retort. To the bottom of this 
flange is attached the cast-iron nozzle 7", which is closed at the 
bottom by the non-return valve /. Underneath the muffles 
are located two hydraulic rams K and a cylindrical iron tank L, 
mounted on wheels, for handling the solid carbonizing material, 
which is in a granular condition. Tank L may be turned on its 
wheels so that spout Q will come under the nozzle / of either 
muffle. 

Operation of the Furnace. — In operating this furnace, a con- 
tinuous method can be employed. When a batch of work has 
been carbonized, the granular carbon is drawn off through nozzle 
/ into tank L. After that, pipe M, through which the carbon- 
izing gas enters the retort, is unscrewed, and ram K raises steel 
pot F towards the top of retort C, thus pushing the work that 
rests on plate G up with it, so that it can be removed as the 
ram proceeds upwards. When casting F has reached the top 



CASEHARDENING 217 

of its stroke, which is a little below the top of the furnace, and 
the old work has been taken out, new work is placed on disk 
G, which is lowered by ram K as fast as the work is located. 
When the retort is filled with work, tank L is wheeled out from 
under the muffles and raised over the top of the furnace with a 
hoist on a swinging arm. When over the top of the furnace, 
pipe N at the bottom of tank L is lowered into opening in the 
center of cover R. Valve P is then opened to allow the hot 
granular carbon to flow out of tank L and fill the interstices 
surrounding the work. 

If granular carbon is held at or near the carbonizing temper- 
ature, it acts very much like a liquid, and readily flows into all 
of the crevices surrounding the articles in the retort that are to 
be casehardened. When it is drawn off at the bottom of the 
retort it is at the carbonizing temperature, and the time con- 
sumed in removing the finished work and replacing it with new 
is so short that the granular carbon does not cool down to a 
temperature below 1500 degrees F. Thus, it retains its mobil- 
ity and flows around the work. An operator on top of the 
furnace might assist this flow by using iron rods that can be 
inserted into the retort through holes in the cover. 

When the retort is properly filled, butterfly valve P is closed 
and tank L is lowered and wheeled to its position underneath 
the muffler. The work is then allowed to stand until the car- 
bonizing temperature has been reached. In the meantime, 
pipe M has been screwed into position, and when the carbonizing 
temperature has been reached, the carbonaceous gas is injected 
into the retort through this pipe. While the work in the retort 
to the left is being carbonized, the retort to the right can be 
emptied and filled, without in any way disturbing the process 
in the other. 

Effect of Compressing the Gas. — Another method that has 
been tested by the designers of this furnace is that of compress- 
ing the carbonizing gases, and some very good results have been 
obtained. The tests demonstrated that when carbon monoxide 
acts on ordinary steel in the presence of free carbon, as in the 
furnace shown in Fig. 1, an increase in the depth of carboniza- 



2l8 



HEAT-TREATMENT OF STEEL 



tion will be obtained 
with an increase of 
the pressure on the 
gas, and there will also 
be an increased con- 
centration of carbon 
within the carbonized 
zone. 

In carrying out 
some experiments of 
this nature, a cylindri- 
cal retort was used, 
like that shown in the 
sectional view in Fig. 
2. In this, electricity 
was used to heat the 
work to the carboniz- 
ing temperature. The 
work was packed in 
charcoal in a retort 
into which a current 
of carbon dioxide was 
injected, in a very 
similar manner to the 
method used in the 
furnace in Fig. i. In 
the illustration, A and 
B are the clamps for 
the terminals and 
these conduct the cur- 
rent to nickel-wire 
spiral D. This wire is 
wound around porce- 
lain tube E, which 
can easily be inserted 



Fig. 2. Carbonizing with Compressed Carbonaceous into, Or taken OUt 01, 




Gas, using Electric Heating Means 



the apparatus. By 



CASEHARDENING 



2ig 



taking off nut N, tube E is readily slipped into fireclay tube 
F, which is surrounded by steel tube G and insulated with 
asbestos. 

The carbon dioxide gas enters the retort through tube C and 
the used gas escapes through pipe H. Porcelain tube / contains 
a thermo-electric couple that is inserted into the retort at the 
opposite end from gas tube C. With it the temperature of the 
entire length of the casehardening chamber can be measured. 
Blocks L are the experimental pieces to be carbonized, and are 

Table II. Results of Carbonizing Steel with Compressed Gas for Three 

Hours 



Kind of Steel 


Carboniz- 
ing Tem- 
perature, 
Degrees F. 


Pounds 
Pressure of 
Carboniz- 
ing gas 


Percentage of Carbon 
at a Depth of 


Mi Inch 


H2 Inch 


Nickel steel* 


1600 

1775 
1650 

I7SO 
1925 
1875 

IS2S 
1650 

1525 


235 
235 
400 

235 
235 
400 

235 
235 
400 


O.71 
O.99 
0.73 

2.22 
3.IO 
2-37 

0.45 
O.76 

0.54 


O.I2 
O.29 
O.36 

I.03 

1-39 
I.40 

0.54 
O.49 
O.56 


Chromium steelf 


Chrome-nickel steel J 





* Composition in per cent: Nickel, 5.02; carbon, 0.118; silicon, 0.20; manganese, 1.53. 

t Composition in per cent: Chromium, 2.33; carbon, 0.41; silicon, 0.15; manganese, 1.02. 

X Composition in per cent: Nickel, 3.17; chromium, 1.5; carbon, 0.33; silicon, 0.06; manganese. 



surrounded by granular carbon. Table II shows some results 
obtained with various kinds of alloy steels. While this is only 
a crude experimental apparatus, it would seem to suggest some 
ideas or principles that can very profitably be used for carbon- 
izing steel parts on a commercial scale. 

Definite proof was obtained that variations in the pressure 
of the carbonizing gas were always accompanied by variations 
in the depth of carbonization, and also in the percentage of 
carbon in the carbonized zone. It was found, however, that 
when the pressure was too high it would cause an oxide to form 



220 HEAT-TREATMENT OF STEEL 

on the steel and this was more pronounced with chromium and 
manganese steels than with others. It was also found that as 
the carbonizing temperature was raised, the pressure could be 
increased without causing this oxide to form. Thus, the higher 
the carbonizing temperature, the higher can be the pressure 
used on the carbonizing gas, with an absolute assurance that no 
oxidation will take place. 

A rod of soft steel, 2§ inches in length and three-eighths inch 
in diameter, was casehardened for about three hours by heating 
three-fourths of an inch of its central portion to about 1800 
degrees F. and allowing this temperature to decrease towards 
the two ends, so that at these the temperature was about 900 
degrees F. Unmistakable carbonizing took place in all portions 
that were above 1450 degrees F. The surface was absolutely 
unaltered in the hottest portion in the center, which was also the 
most intensely carbonized, while a distinct layer of oxide was 
seen in the cooler portions, this oxide thickening as the temper- 
ature lowered towards the ends. 

Such good results were obtained by compressing the carbon- 
izing gas as it was injected into the bed of charcoal in which 
the work was packed in the retort, that this method promises to 
become a commercial success. While the mixed agent, carbon 
monoxide and charcoal, increased both the speed of penetration 
and the percentage of carbon in the carbonized zone over all 
previous methods or materials used for carbonizing steels, com- 
pressing the carbon monoxide has still further increased these 
factors. Like all methods and processes, however, it must be 
handled properly. The amount of compression, as well as the 
carbonizing temperature, varies with different kinds of steels. 
Therefore, these must be discovered and properly adjusted, if 
work is to be turned out that is free from oxide and scale, and 
that has the desired penetration uniformly distributed over all 
portions of the exposed surfaces. 

Whether work is carbonized with an ordinary flow of carbon 
dioxide into and through the charcoal, or by compression, the 
advantages which vertical muffles, as shown in Fig. 1, have over 
horizontal muffles are in the greater speed of charging and re- 



CASEHARDENING 221 

moving the work, due to the greater simplicity of the operations, 
the uniformity of the treatment of all pieces forming the charge, 
and the more uniform distribution of the carbonizing gases, due 
to the spaces being reduced to a minimum. 

Time Required for Operation. — The time for the various 
operations with the furnace shown in Fig. i is as follows : Charg- 
ing the pieces to be carbonized, from i to 5 minutes, according 
to their size and shape; completely filling the retort with granular 
carbon, from 1^ to 4 minutes; lowering ram K, replacing pipe 
M, removing tank L, and closing down cover R, 1 minute; 
drawing the granular carbon from retort C into tank L, 4 min- 
utes; raising ram K and removing the work from the retort, 
2 minutes. The time consumed in all the operations, where 
ordinary work is being handled would, therefore, be about ten 
minutes, but with specially shaped pieces and unfavorable con- 
ditions, this time might be extended to 30 minutes. 

By pre-heating the work to a carbonizing temperature before 
putting it into the retort, no time will be lost in fully heating it 
in this furnace. The temperature of the granular carbon can 
then be maintained nearly up to the carbonizing temperature, 
as it will not be chilled by cold work. The process can thus be 
made strictly continuous. Under these conditions, a depth of 
carbon penetration of g 1 ^ inch can be given the work in one 
hour, and of jq inch in two hours. Thus it will be seen that 
from i\ to 2 J hours is all that is required for the complete car- 
bonizing operations in one retort. 

By using gas for fuel and gas for carbonizing, the work can 
be controlled within closer limits than with any other process, 
unless it should be an electric one, and the arrangements of 
this furnace are such that its capacity for producing work is 
greater than that of any furnace which has been designed with 
the same size of work holder. If there is not enough work to 
keep both muffles going on pre-heated work, one of the muffles 
can be used as a pre-heating furnace, while the other is doing 
the carbonizing. By alternately using the muffles for pre- 
heating and carbonizing, an amount of work will be turned out 
that will compare favorably with any other casehardening 



222 HEAT-TREATMENT OF STEEL 

furnace. If desired, the current of carbonaceous gas can be 
used for a whole or any given part of the carbonizing time, and 
thus the results obtained can be made to cover a wide range. 
Where localized casehardening is required, the granular carbon 
can be drawn off until only enough is left to insure the chemical 
equilibrium in the gas, and by thus isolating the carbon monox- 
ide, it will intensify its specific action. 

Apparatus for Casehardening by Gas, Suitable for Small and 
Medium Work. — The casehardening process described on the 
preceding pages is intended for specially large work. The 
American Gas Furnace Co. has brought out a casehardening 
plant, using gas as the carbonizing material, which is suitable for 
small and medium work. Briefly stated, the process as per- 
formed by the apparatus brought out by this company consists 
in placing the work in a slowly revolving, properly heated, 
cylindrical retort into which the carbonizing gas is injected 
under pressure. From the gas, the work absorbs the volatile 
carbon. The absorption of carbon begins as soon as the work is 
sufficiently heated to attract it, and continues throughout the 
process, because the work is constantly and uniformly exposed 
to a carbon charged atmosphere under pressure, instead of to 
solid carbonaceous material which turns to ashes wherever it is 
in proximity to the heated parts. All the parts of the same 
piece of work and all the pieces contained in one charge in the 
retort are, therefore, continuously subjected to exactly the same 
condition as regards the presence of carbon, and the result is 
a uniformity and speed of operation not obtainable by other 
methods. 

The complete gas casehardening plant consists of a generator 
of carbonizing gas and revolving retorts used for the carboniz- 
ing process. The generators are generally made in sizes to supply 
two or more machines with carbonizing gas. The gas is pro- 
duced from refined petroleum and the carbon vapor is so diluted 
by a neutral gas that the proportion of carbon that is supplied 
to the work is not greater than that which can be absorbed by 
the work without forming obstructive carbonaceous deposits. 
The carbonizing machine proper consists of a carbonizing retort 



CASEHARDENING 



223 




224 HEAT-TREATMENT OF STEEL 

and a cylindrical furnace body, in which the retort is enclosed 
and in which it rotates. Suitable arrangement is made for 
charging and discharging the work. The furnace for the exterior 
heating of the retort is fired with fuel gas requiring a positive 
air blast. Besides these carbonizing retorts, of course, ordinary 
furnaces for reheating the work for hardening are required. This 
reheating can also be done in the carbonizing retort, if desired. 

The line-engraving, Fig. 3, shows a sectional view of the car- 
bonizing machine. A wrought-iron retort is shown at A which 
is slowly rotated on rollers B by worm-gear C, which, in turn, is 
driven by worm D, the shaft of which is rotated in any suitable 
manner, preferably by a sprocket and chain. At E are shown 
air spaces in the retort formed by two pistons I between which 
the work is placed. At F is shown the heating space surround- 
ing the retort into which the fuel gas and air are injected under 
pressure from two rows of burners, indicated in the upper half 
of the casting at G. The cover H closes the retort. It is con- 
nected to the piston / by pipe /, which also provides a vent for 
the retort. Cover H and this pipe are withdrawn to charge 
and discharge the retort, and are replaced after the work is 
inserted, before beginning the carbonizing process. 

Steel to Use for Gas Casehardening. — For casehardening 
by the gas method, it has been found that articles made from 
machine steel containing from 0.12 to 0.15 per cent carbon 
give the best results, although steel containing from 0.20 to 0.22 
per cent carbon may also be used to advantage. The length of 
time that the work is required to remain in the carbonizing retort 
depends upon the depth of carbonized surface required. A 
thin shell will be produced in one hour, while the thickness will 
constantly increase if the work is left in the retort up to nine or 
ten hours. The treatment after the work is carbonized is the 
same as that which should be given to ordinary casehardened 
work. As already stated, it is rarely the case that work is prop- 
erly hardened, if quenched directly from the carbonizing retort, 
but, as a general thing, it should be allowed to cool slowly and 
then be reheated to harden the carbonized surface at the proper 
hardening heat. 



CASEHARDENING 225 

The heat of the retort while carbonizing the work must be 
varied for different classes of steels, and the proper degree can 
only be determined by trial. The higher the heat, the quicker 
the carbon will be absorbed from the carbonaceous gas, but the 
higher heat tends to make the core coarse. As a rule, about 
1500 degrees F. will be found a suitable temperature, and this 
should not be exceeded unless tests have been made to deter- 
mine that higher temperature may be used without detriment 
to the structure of the steel. 

The gas casehardening process can be carried out more rapidly 
and more uniformly than is possible with solid carbonaceous 
materials. Another advantage is that the volatile carbon will 
find its way into slots, holes and cavities which could not receive 
the carbon from the granulated bone or any other solid packing 
material, and, hence, the uniformity of the product is greater. 
In many cases, low-carbon steel treated by the gas caseharden- 
ing process may, therefore, be substituted for tool steel in ma- 
chine construction. 



CHAPTER XII 
HEAT-TREATMENT OF GEARS FOR MACHINE TOOLS 

In the earlier days of machine tool construction, when carbon 
tool steel was used for cutting, and relatively light work was 
the order of the day, cast-iron gears were used for transmitting 
power. With the advent of air-hardening tool steel and heavier 
work, the use of mild steel gears became necessary, while to-day, 
with tools of modern high-speed steel the use of heat-treated 
alloy steel gears is well nigh imperative. Gears of this last class 
may be divided into two general groups — casehardened gears, 
with a low-carbon soft center or core and a high-carbon hard 
exterior or case, and hardened high-carbon gears which are of 
the same composition and hardness throughout. The charac- 
teristics, heat-treatments and merits of these two groups, as 
viewed in the light of a wide experience with gears used in motor- 
car construction, will be discussed briefly in the following. 

Casehardened Gears. — Case-carbonized gears may be made 
from four general classes of steel, viz., straight-carbon, nickel, 
chrome-vanadium and chrome-nickel steel, and of each of these 
classes several modifications will be found in the market. On 
the whole, the steels containing chromium are to be preferred, 
for they are freer from the tendency to lamination shown in 
nickel-steel (especially 3J per cent nickel steel) and they also 
absorb carbon more easily, thereby lessening the length of time 
and expense of carbonizing. Before carbonizing, the carbon 
content of each of the steels mentioned, should be about 0.20 
per cent, and never more than c.25 per cent, to avoid brittleness 
in the teeth. The carbon in the case should be raised to about 
0.90 per cent, which can readily be done by the proper selection 
of carbonizing material and by using the proper temperature 
for carbonizing. 

The temperature for carbonizing, in general, should be about 

226 



HARDENING GEARS 227 

1600 to 1650 degrees F. for all the classes of steel previously 
referred to. Lower temperatures do not give sufficient depth 
of " case," unless the heating operation is much prolonged. On 
the other hand, higher temperatures result in a case of exces- 
sive carbon content and in a core of such large-grained size that 
it will not respond to the subsequent heat-treatment as readily 
as if a temperature of 1600 to 1650 degrees had been used. 

The heat-treatment after case-carbonizing is the most impor- 
tant part of the process, and upon it depend the physical prop- 
erties of the finished work. As already stated, after carbonizing 
we have a piece of steel with a 0.20 per cent carbon core, and a 
0.90 per cent carbon case, and the object of the treatment is to 
put both the core and the case into the best possible physical 
condition. Both need refining to correct the large-grained 
structure developed by subjecting the steel for many hours to 
the carbonizing temperature. Since the refining or hardening 
temperature of the core is about 200 degrees F. above that of 
the case, this difference determines the most approved method 
of heat-treatment. 

The old method consisted in quenching the piece in oil or 
water at the end of the carbonizing operation, right from the 
box, at the temperature used for carbonizing. This resulted 
in a large-grained core that was neither strong nor tough, and 
an overheated granular case which was hard, but which would 
not stand up in service any better than an overheated piece of 
tool steel. The first improvement on this old method was to 
allow the piece to cool in the box after it was removed from the 
carbonizing furnace, and then to reheat it to the proper temper- 
ature for hardening the case and quench in a suitable fluid. 
This procedure, however, did not develop a strong tough core. 

The proper heat-treatment for casehardened gears is the so- 
called double treatment by which the pieces are first allowed to 
cool in the box after carbonizing, next reheated to from 1550 to 
1625 degrees F. and quenched in a suitable medium to refine 
the core, then reheated to from 1350 to 1425 degrees F. and 
again quenched in a suitable medium to harden the case, and 
finally drawn in oil at not above 400 degrees F. to further 



228 



HEAT-TREATMENT OF STEEL 



increase the strength and toughness of the casehardened gear. 
The temperatures given are approximate only; for exact infor- 
mation concerning any particular steel, the user should con- 
sult the steel-maker. 

There are many case-carbonizing compounds on the market 
and most of them have some merit. Those of bone are probably 
the least desirable owing to their lack of uniformity which 
results in uneven carbonizing. The most desirable are those 
consisting of definite mixtures of carbon and carbonates; they 
carbonize uniformly, and most of them can be used repeatedly 
without losing their power of giving up carbon to the metal. 

The wear and tear on carbon- 
izing furnaces, the fuel consumed, 
and the expense of the boxes are 
three important items in the cost 
of casehardening. In many cases 
it is possible to reduce all these 
items by the use of a cored instead 
of a solid box, as shown in Fig. i. 
The proportion, of course, will 
vary with the work to be done, 
but if the general idea is worked 
out for each specific instance, it 
will be found not only that the 
cost of carbonizing is diminished, but also that the carbonizing 
is more uniform. 

Hardened High-carbon Gears. — Unlike casehardened gears, 
hardened high-carbon gears are of uniform carbon content 
throughout, and, when hardened, have a uniform hardness 
throughout the tooth-section. The steels used for these gears 
are of three general classes, viz., silico-manganese, chrome- vana- 
dium and chome-nickel steel — the last-named, in its several 
modifications, being by far the most used. The carbon content 
varies for the different classes from 0.40 per cent to 0.60 per cent. 
The heat-treatment of all these steels is very simple, consisting 
merely in heating the gear slowly and uniformly to the harden- 
ing temperature, which is usually about 1500 degrees F., quench- 




Machinery 



Fig. 1. 



Cored Pot or Box for 
Casehardening 



HARDENING GEARS 



229 



ing in oil, and afterward drawing in an oil bath. The result 
is strong, tough, dense-grained steel gears; these have been used 
with marked success in motor-car work, and are fast replacing 
soft steel and casehardened gears in machine-tool construction. 
Viewed from the standpoint of physical properties in the 
finished gear, the evolution in gear material from cast iron to 
tempered steel, may be seen in the following figures: 



Material 


Elastic Limit, 

Pounds per 

Sq. In. 


Hardness — 
Scleroscope 


Toughness — 
Guillery Impact 
Kilogrammeter 


Cast iron 

Soft steel 


20,000 
40,000 

120,000 

225,000 


25 

35 
35 
75 


Negligible 
2 

2-5 

5 


Casehardened steel; (average test 
of alloy steel) 


Tempered steel; (average test of 
alloy steel) 





Comparison of Results. — For machine tools, hardened high- 
carbon alloy-steel gears appear to be preferable to casehardened 
gears for a number of reasons: 

1. Physically they are stronger and tougher and should 
therefore be better able to resist sudden impacts and extraor- 
dinary loads. They do not show by fileand scleroscope test the 
same degree of hardness as casehardened gears, but, neverthe- 
less, with proper design, the dense-grained gear-tooth resists 
wear most satisfactorily, as was demonstrated recently by the 
examination of a motor-car transmission that had covered over 
100,000 miles. The high-carbon steel gears in this car still 
showed the original tool-marks. Not long ago a designer of 
machine tools commented on the apparent softness of some 
hardened high-carbon gears, but found after several months of 
hard service that they still showed tool-marks, thus proving 
hardness ample for wear. 

2. In service, especially for " clash gears," the superiority of 
these gears is most marked. On the clashing faces, casehard- 
ened gears are likely to have the hard case chipped off, thereby 
exposing the soft core to the impact of clashing. The hard 
chips fall into the gearing and may find their way into bearings, 



230 HEAT-TREATMENT OF STEEL 

thus causing trouble. High-carbon steel gears with a uniform 
hardness throughout do not chip, nor do they " dub over." 

3. The heat- treatment of high-carbon steel gears is much 
simpler than that required for proper casehardening. It is 
shorter, less costly and produces a more uniform product, and 
as the gear is heated but once for hardening, as compared with 
three times for casehardening, the finished gear is certain to be 
freer from warpage. The cost of proper casehardening is not 
generally appreciated, but it has been found that a caseharden- 
ing steel must cost three to four cents per pound less than a 
regular high-carbon hardening steel, if finished gears made from 
both materials are to cost the same. 

With all heat-treated gears, little points in design are impor- 
tant. The gear-teeth should not be undercut, for if the section 
at the root-line is smaller than at the pitch-line, greater hard- 
ness and brittleness is produced where least desired. Great 
differences in section should be avoided wherever possible, so as 
to do away with excessive warpage. Sharp edges and angles, 
even in keyways, are the cause of internal hardening strains 
which frequently result in failures; hence, wherever possible, a 
fillet should be used in place of a sharp angle. 

Furnace for Heat-treating Gears. — When heat-treated gears 
are suggested to the machine-tool builder as a remedy for some 
of his troubles, and as a means of eliminating an excessive item 
for replacements and repairs, one of his first questions natur- 
ally is, of what does a heat-treating equipment consist? Usu- 
ally the second question is, what will it cost? The first item 
in equipment is a furnace. There are offered for sale a number 
of types of gas- and oil-fired furnaces, but few are located in 
natural gas districts, and the price of fuel-oil has almost driven the 
oil-fired furnace from economic use. Coal and coke, however, 
are available everywhere, and a furnace using fuel of this kind 
is shown in Fig. 2. Anthracite, bituminous coal, and coke work 
equally well. When bituminous coal is used, the consumption 
with ordinary firing should not exceed 500 pounds in a twenty- 
four hour day. The cost depends somewhat upon the price of 
labor, but should not exceed $300. 



HARDENING GEARS 



231 




Machinery 



Fig. 2. Coal or Coke Furnace for Heat-treating Gears 



232 HEAT-TREATMENT OF STEEL 

Quenching Baths. — The next item is a proper quenching 
medium. Running water with a suitable tank is always neces- 
sary in a hardening room. To take the chill from the water in 
winter, or to raise the temperature a little at any time, a jet of 
live steam in the incoming water pipe will be found very con- 
venient. The heat-treatment of alloy gear-steels requires an 
oil bath, the size of which depends entirely upon the amount of 
work to be quenched and the facilities for keeping the oil cool. 
The kind of oil best suited for oil-hardening was the subject of 
an investigation conducted by the laboratory of the Carpenter 
Steel Co., with the following results, comparison being made 
with water as a standard: 

Hardening Hardening 

Medium Quality 

Water i . ooo 

Mineral No. i o . 2409 

Mineral No. 10 o . 2304 

Corn 0.1927 

Mineral No. 2 o . 1607 

Cotton-seed o . 1606 

Fish o . 1490 

Rosin o . 1350 

For hardening, several of the mineral oils are more effective 
than fish and cotton-seed oils, which for a long time were looked 
upon as the best oils for this purpose. Mineral oil No. 1 has a 
specific gravity of 0.860, a flash point of 420 degrees F., and a 
viscosity of 170 seconds at 100 degrees F., as shown by the 
Saybold viscosimeter. A mineral oil to this specification can 
be bought very cheaply. 

Oil can be cooled by blowing cold air through it, or by pump- 
ing the oil through a coil of pipe immersed in cold running 
water, thus maintaining a circulatory system which admits 
cool oil at the bottom of the hardening tank and pumps the 
warm oil from the top through cooling coils back to the bottom. 
When air is used, care should be taken to avoid an excess during 
the quenching of a piece, for if air instead of oil were to strike 
the piece constantly, uneven hardening might result. 



HARDENING GEARS 233 

Drawing or Tempering Bath. — The next equipment item is a 
drawing bath. This may consist of oil, lead, or a combination 
of salts, contained in a cast-iron or steel vessel. The container 
is usually of very simple design and may be fired by gas, oil or 
coal. The oil should be a mineral oil with a flash point of not 
less than 600 degrees F., this temperature usually being sufficient 
for all temper-drawing purposes. If higher temperatures are 
desired, a mixture of two parts potassium nitrate and three parts 
sodium nitrate may be used. This mixture melts at 450 de- 
grees F. and may be used for temperatures up to 1000 degrees 
F., or lead, which melts at 630 degrees, may be substituted. 
To indicate the temperature of the bath, a mercury thermom- 
eter should be used rather than a pyrometer, for most pyrometers 
will show considerable error at drawing temperatures under 
800 degrees F. 

Application and Calibration of Pyrometers. — The last item 
is a pyrometer. There are a number of good thermo-electric 
pyrometers on the market, and more depends upon the care of 
the instrument than upon the selection of any particular make. 
Following are a few rules for the use of the pyrometer and a 
simple method of calibration: 

1. Keep the hot end of the thermo-couple as near the work 
as possible; do not put it through the furnace wall or roof, 
exposing the end to the direct heat of the flame, but place it so 
that it will attain, as nearly as possible, the same temperature 
as the work. 

2. Keep the cold end of the thermo-couple protected from the 
direct or radiating heat of the furnace; that is, keep it cool. 

3. Protect the voltmeter by a dust-proof case, and place it 
on a support free from vibration. 

4. All switches should be of the wiping-knife type. Improper 
contact at the switches is a prolific source of error, and such 
errors are not readily located. 

5. Carefully check all thermo-couples as soon as they are 
received from the manufacturer and before putting them into 
service. Adhere closely to this rule, instead of assuming that 
new thermo-couples are sure to be correct. New thermo-couples 



234 HEAT-TREATMENT OF STEEL 

should not be used on faith without checking, since they occa- 
sionally show a considerable error, and any one making use of 
them as standards will sooner or later come to grief. 

6. Carefully standardize each pyrometer at definitely stated 
intervals — at least once a week, and as much oftener as pos- 
sible. Frequent calibration is a matter not of convenience, but 
of necessity. 

How can the calibration of a pyrometer be accomplished 
readily and accurately without the use of an extensive labora- 
tory equipment? To this question of immediate interest in 
every hardening-room, the answer is that the easiest and most 
convenient method is based upon determining the melting point 
of common table salt (sodium chloride). Chemically pure salt, 
which is neither expensive nor difficult to procure, should be 
used where accuracy is desired. The salt is melted in a clean 
crucible of fireclay, iron or nickel, either in a furnace or over 
a forge fire, and is then further heated until a temperature of 
about 875 degrees to 900 degrees C. (1607 to 1652 degrees F.) is 
attained. It is essential that this crucible be clean, because a 
slight admixture of a foreign substance might noticeably lower 
or raise the melting point. The thermo-couple to be calibrated 
is then removed from its protecting tube and its hot end is im- 
mersed in the salt bath. When this end has reached the tem- 
perature of the bath, the crucible is removed from the source 
of heat and allowed to cool, and cooling readings are taken every 
ten seconds on the voltmeter. A curve is then plotted by using 
time and temperature as coordinates, and the temperature of 
the melting point of salt, as indicated by this particular thermo- 
couple, is noted at the point where the temperature of the bath 
remains temporarily constant while the salt is freezing. The 
length of time during which the temperature is stationary de- 
pends on the size of the bath and the rate of cooling, and is not 
a factor in the calibration. The true melting point of salt is 
801 degrees C. (1474 degrees F.), and the needed correction for 
the instrument under observation can be readily applied. 

Cost of Equipment. — The cost of this equipment, including 
a coal-fired furnace, as shown in Fig. 2, five to seven barrels of 



HARDENING GEARS 235 

hardening oil, one barrel of drawing oil, the tanks for holding 
these oils and a pyrometer, should be about $500 to $600. The 
equipment just noted is the one necessary for tempered gears. 
When casehardened gears are heat-treated, there is necessary, in 
addition to this, case-carbonizing boxes and a carbonizing com- 
pound. A second furnace may also be necessary, depending 
upon the quantity of casehardened gears to be treated. It is thus 
seen that for casehardened gears, the heat-treating equipment is 
more expensive than that required for tempered gears. 

Heat-treated gears appeal to the progressive machine-tool 
builder. They will make possible the use of gears of smaller 
section, and while this may not be necessary from the standpoint 
of weight, as is the case with the motor-car builder, an economy 
of space is frequently desirable. The greatest advantage is, 
perhaps, the elimination of repairs and replacements. 

The information contained in the foregoing, relating to heat- 
treatment of gears, is abstracted from a paper read by Mr. J. H. 
Parker, before the National Machine Tool Builders' Association. 



CHAPTER XIII 
TESTING THE HARDNESS OF METALS 

Importance of Hardness Tests. — Few properties of iron and 
steel are of more importance than that of hardness. In some 
cases, as with a cutting tool or a pressure die, the metal is prac- 
tically valueless unless it can retain a sharp edge; while in other 
instances, where the material has to be machined or cut or trued 
to shape, even a relatively slight increase of hardness is the 
cause of much inconvenience and expense. In a third class of 
material a good wearing surface is of prime importance; while, 
lastly, hardness may often serve as an indication of a degree 
of brittleness and untrustworthiness which might perhaps be 
otherwise unsuspected. 

Definition of Hardness. — Hardness may be defined as the 
property of resisting penetration, and, conversely, a hard body 
is one which, under suitable conditions, readily penetrates a 
softer material. There are, however, in metals various kinds 
or manifestations of hardness according to the form of stress 
to which the metal may be subjected. These include tensile 
hardness, cutting hardness, abrasion hardness, and elastic hard- 
ness; doubtless other varieties could also be recognized when 
the experimental conditions are modified so as to bring into 
operation properties of the material in addition to that of simple, 
or what may be conveniently called mineralogical hardness. 
This has been defined by Dana as " the resistance offered by a 
smooth surface to abrasion." 

The usual quantitative tests for hardness are static in char- 
acter, but the conditions are profoundly modified when the 
penetrating body is moving with greater or less velocity. The 
resistance to the action of running water, to the effect of a sand- 
blast, or to the pounding of a heavy locomotive on a steel rail, 
affords examples of what might perhaps for purposes of distinc- 

236 



HARDNESS TESTING 237 

tion be called dynamic hardness, which is a branch of the sub- 
ject that has received little attention. 

Simple Methods for Testing Hardness. — Even when men 
first began to harden steel, they probably sought some method 
of ascertaining in particular cases whether their object had been 
accomplished. Perhaps the testing tool was nothing more than 
a fragment of flint or another piece of steel known to be hard. 
Certain jewels — as the diamond — are well suited to a process 
which depends upon scratching. In fact, this process is in com- 
mon use everywhere even at the present day. 

The test by riling is not to be despised as it is easily applied, 
and if the file is a good one, the results are sufficiently accurate 
and reliable for a considerable class of work. But the file is an 
instrument inadequate to the requirements of modern metallur- 
gists and manufacturers. This is true for two reasons: First, 
the alloy steels seem to possess the property of being able to 
resist a file, apart from hardness. Thus, a piece of manganese 
self-hardening tool steel may be, in reality, softer than a speci- 
men of a pure carbon steel, and yet resist the attacks of the file 
equally well. 

In explanation of this phenomenon, it has been suggested 
that the hard manganese resists the file while the iron substratum 
remains soft. The combination as a whole would not be so hard, 
although able to withstand the file. This, however, seems 
really to involve the proposition that such steel is not a perfect 
chemical combination, but that particles of manganese are held 
imbedded in iron or an iron alloy. Perhaps this may be so, 
but if it is true, then the action of such steel on the file is very 
similar to that of an emery wheel. The emery itself is very 
hard, but is held in a matrix that is soft. However, whether we 
accept this explanation or not, it is doubtful whether we have 
good reason to contend that a specimen of alloy steel is as hard 
as a piece of pure carbon steel, merely because it resists the file 
equally well. 

The second objection to the file is that it affords no reliable 
means of making accurate comparisons between different de- 
grees of hardness. It is sometimes of importance in cases where 



238 HEAT-TREATMENT OF STEEL 

one element of a machine slides against another to ascertain which 
of the two is the harder. The difference may be very slight, yet 
it will readily be granted that this difference might become of 
importance if lubrication failed, for the harder piece would then 
cut or wear the softer. If such a contingency is possible, then 
it is important that the more expensive part shall be the harder. 
A little reflection will convince one that this principle of associat- 
ing a harder valuable part with a softer less valuable part has 
application everywhere in machine construction; but in order 
to apply this principle widely, it is necessary to be able to deter- 
mine differences in hardness where these differences are quite 
small in amount. 

Modern Methods for Testing Hardness. — Comparison will 
be made in the following of four typical methods of measuring 
hardness. Those selected include the sclerometer, introduced 
by Prof. Thomas Turner in 1886; the scleroscope, introduced in 
1907 by Shore; the form of indentation test adopted by Brinell 
about 1900; and the drill test introduced by Keep in 1887. 
Each of these methods has been used in actual works practice, 
and by various persons other than the inventor, and may thus 
be regarded as being typical of the particular class of test to 
which it belongs. Among the many other forms of test, the 
microsclerometer and wearing tests call for special mention, 
though to these only incidental reference can be made. The 
principles underlying the four methods selected for comparison 
will be briefly described in the following. 

Turner's Sclerometer. — In this form of test a weighted dia- 
mond point is drawn, once forward and once backward, over 
the smooth surface of the material to be tested. The hardness 
number is the weight in grams required to produce a standard 
scratch. The scratch selected is one which is just visible to the 
naked eye as a dark line on a bright reflecting surface. It is also 
the scratch which can just be felt with the edge of a quill when 
the latter is drawn over the smooth surface at right angles to a 
series of such scratches produced by regularly increasing weights. 

Shore's Scleroscope. — In this instrument, which will subse- 
quently be described in detail, a small cylinder of steel, with a 



HARDNESS TESTING 239 

hardened point, is allowed to fall upon the smooth surface of 
the metal to be tested, and the height of the rebound of the 
hammer is taken as the measure of hardness. The hammer 
weighs about 40 grains, the height of the rebound of hardened 
steel is in the neighborhood of 100 on the scale, or about 6| 
inches, while the total fall is about 10 inches or 255 millimeters. 

Brinell's Test. — In this method, described in detail in suc- 
ceeding pages, a hardened steel ball is pressed into the smooth 
surface of the metal so as to make an indentation of a size such 
as can be conveniently measured under the microscope. The 
spherical area of the indentation being calculated, and the pres- 
sure being known, the stress per unit of area when the ball comes 
to rest is calculated, and the hardness number obtained. With- 
in certain limits the value obtained is independent of the size of 
the ball, and of the amount of pressure. In the original tests 
the steel ball was 10 millimeters (0.394 inch) in diameter, and the 
pressure was equal to a weight of 3000 kilograms; but a more 
convenient form of apparatus is now supplied by Mr. Brinell 
for works tests, while Mr. Stead and Mr. Derihon have intro- 
duced small portable instruments. 

Keep's Test. — In this form of apparatus a standard steel 
drill is caused to make a definite number of revolutions while 
it is pressed with standard force against the specimen to be 
tested. The hardness is automatically recorded on a diagram 
on which a dead soft material gives a horizontal line, while a 
material as hard as the drill itself gives a vertical line, inter- 
mediate hardness being represented by the corresponding angle 
between o and 90 degrees. 

Comparison between Testing Methods. — Each form of test 
has its advantages and its limitations. The sclerometer is 
cheap, portable, and easily applied, but it is not applicable to 
materials which do not possess a fairly smooth reflecting surface, 
and the standard scratch is only definitely recognized after some 
experience. The Shore test is simple, rapid, and definite for 
materials for which it is suited, and appears likely to have an 
important future; but further information is yet needed as to 
the exact property which is measured bv this form of test. As 



240 HEAT-TREATMENT OF STEEL 

shown by De Freminville, the result obtained varies somewhat 
with the size and thickness of the sample, while if the test-piece 
js supported on a soft material, such as a plasticine, the results 
are valueless. It should also be pointed out that india-rubber 
gives a rebound of 23, which is equal to that of mild steel, while 
light soft pine wood gives a rebound of 40, which is nearly twice 
as great as that of gray cast iron. Curiously enough, hard wood, 
like teak, gives a rebound of about 12, while some samples are 
considerably lower than this. 

As illustrating the influence of the support, a sample of ex- 
ceptionally hard rolled copper, about 0.040 inch in thickness, 
when supported on a block of hard steel, and tested with the 
blunt or " magnified " hammer supplied, gave a value of 30, 
which was increased to 34 when the copper was supported on 
wood. A sample of brass only gave a value of 17, and yet this 
brass would scratch the copper, while the copper would not 
scratch the brass. From these results it would seem that the 
Shore test is only applicable to a certain class of substances. 
It appears to test what may be termed the " elastic hardness," 
and gives high results with metals in the " worked hard " condi- 
tion. Tests appear to show that good results are, however, 
obtained with glass and with porcelain, as well, of course, as with 
most metals. 

The Brinell test is especially useful for constructive material; 
it is easily applied and definite, and is now of all hardness tests 
the one most employed. It appears to give satisfactory results 
with wood, but cannot be applied to very brittle materials, such 
as glass, or to hard minerals. Keep's test is especially suited for 
castings of all kinds, as it records not merely the surface hard- 
ness, but also that of the whole thickness, and gives indications 
of blowholes, hard streaks, and spongy places. Obviously, it 
can only be applied to materials the hardness of which is less 
than that of hardened steel. 

Comparison of Results Obtained by Different Testing Methods. 
— A very important question arises in connection with these 
various tests — namely, as to whether there is any observed 
agreement between the results which are arrived at by such 



HARDNESS TESTING 



241 



entirely different methods. It will be noticed that in each case 
an arbitrary scale is adopted. If the weights used on the scle- 
rometer had been ounces instead of grams, the hardness numbers 
would naturally have been different. Similarly, Brinell's tests 
might have been expressed in tons and inches, or a different 
weight of hammer and height of scale adopted by Shore. Hence 
all that can be expected is a proportionality in the results, and 
if this is ascertained it should be possible to convert values on 
one scale into results on another. 

An examination of results obtained by the four methods dealt 
with shows that, for relatively pure metals in their cast or normal 
condition, there is a general agreement which must be regarded 

Table I. Hardness Scales Compared 



Metal 


Sclerometer 


Scleroscope 


Brinell 


Lead 

Tin 

Zinc 


I.O 

2-5 

6.0 
8.0 

150 

21.0 

21-24 

24.0 

36.0 

72.0 


I.O 

3-o 

7.0 

8.0 

12.0 

22.0 
24.0 
27.0 
40.0 
70.0 
95o 


I .O 
2-5 

7-5 

12.0 

14.5 

16-24 

24.0 

26-35 

35-o 

75o 

93 


Copper, soft 


Copper, hard 


Softest iron 


Mild steel 


Soft cast iron 


Rail steel 


Hard cast iron 


Hard white iron 


Hardened steel 





as remarkable. In Table I will be found, in the first column, 
results which were published by Prof. Turner in a paper on the 
hardness of metals in 1886. In the second column are Prof. 
Turner's results with the Shore scleroscope, and these figures 
are in good agreement with those supplied by the maker of the 
instrument. In the third column are values taken from pub- 
lished results by Mr. Brinell and by Mr. Stead, but the numbers 
given have been divided by 6, as this figure has been found 
to suitably reduce the Brinell hardness values for purposes of 
comparison. 

It will be observed that either by accident or design the scale 
adopted for the scleroscope is, for practical purposes, identical 



242 HEAT-TREATMENT OF STEEL 

with that of the sclerometer, while Mr. Brinell's values are 
proportional. The angles in Keep's tests could easily be made 
to show pretty close agreement with the other values. It would 
therefore appear that each instrument, with simple and homo- 
geneous substances, must measure one and the same physical 
property, and give results which are either in actual agreement 
with, or proportional to, the results obtained by the other forms 
of tests. 

Hardness Tests on Worked or Heat-treated Metals. — In 
practice, however, the use of relatively pure metals in the un- 
worked or annealed condition is comparatively rare, unless we 
include in this category wrought iron and mild steel. Carbon 
steels and special steels consist largely of alloys, the complexity 
of which is profoundly modified by heat- treatment; while copper, 
zinc and their alloys are frequently hardened by rolling, draw- 
ing or other mechanical treatment. 

The very important question therefore arises as to the extent 
to which the different methods of testing agree in their values 
for hardened and tempered steel, and for the hardness caused 
by mechanical treatment. From preliminary observations on 
the latter point Prof. Turner states as his belief that metal which 
has been mechanically treated, as with hard-drawn rods or rolled 
sheets, has its tenacity increased out of proportion to its hardness 
as measured by a file or cutting tool. The sclerometer shows 
relatively little difference, for example, between hard-drawn and 
annealed copper, while the scleroscope shows an exaggerated 
effect, at all events in some cases. As the Brinell test closely 
follows the tenacity, it too may be expected to show a marked 
difference between worked and annealed samples. The result 
in some cases is likely to be a confusion between elasticity or 
tenacity on the one hand, and true or mineralogical hardness 
on the other. For example, a piece of hard-rolled copper may 
give a greater hardness number than one of mild steel; yet a 
tool made of mild steel will always cut copper, but no amount 
of cold-rolling will make copper cut steel. Hence great care is 
required when hardness values for different materials are com- 
pared. 



HARDNESS TESTING 



243 



Hardness of Steel in Hardened, Tempered or Annealed Con- 
dition. — The question of agreement in reference to the true 
hardness of a sample of steel in the normal, hardened, tempered, 
or annealed condition is perhaps of even greater importance. 
To illustrate the kind of difficulty which arises, reference may 
be made to some recently published results by E. Maurer, in 
which samples of steel with varying content of carbon were 
heated to ascertained temperatures, quenched, and afterward 
tempered or annealed at given temperatures. The hardness of 
the samples was then determined. When the tempering heat 
was 300 degrees C, the loss of hardness in a sample containing 
0.83 per cent of carbon was n.i per cent by the Shore method, 

Table n. Percentage of Loss of Hardness of Hardened Steel when Tem- 
pered to Various Temperatures, as Measured by Different Hard- 
ness Testing Apparatus 



Temperature 


Brinell Method 


Martens Sclerom- 


Jagger Microscle- 


Shore's Method 


of Heating, 


(0.83 per cent 


eter (0.95 per cent 


rometer (0.86 per 


(0.83 per cent 


Degrees C 


Carbon) 


Carbon) 


cent Carbon) 


Carbon) 


160 




2-5 


1.8 


3-7 


200 


13 


14.0 


5-4 


2.7 


300 


38 


41 .0 


9.1 


11. 1 


400 


68 


70.6 


23.6 


33 0* 


SOO 


94 


87.5 


64.0 


92. 5 


600 


100 


95-7 


94- 5 


100. 



* At 380 degrees C 

and 38.0 per cent by the Brinell test. A steel with 0.95 per cent 
carbon tested in a similar manner by Heyn and Bauer with a 
Martens sclerometer gave a loss of hardness of 41.0 per cent; 
while lastly Boynton, with a Jagger sclerometer, using a steel 
with 0.86 per cent carbon, has recorded a loss of hardness on 
tempering at 300 degrees of only 9.0 per cent. 

The question may be put in this way: The steel is suited for 
making woodworking tools, if properly hardened and tempered; 
is 300 degrees C. a proper tempering heat? According to the 
Shore test and the Jagger test the tool should be hard and cut 
well; but according to the Brinell test and the Martens sclerom- 
eter it has lost nearly half its original hardness, and should 
rapidly lose its cutting edge. Maurer states that everyday 



244 



HEAT-TREATMENT OF STEEL 



experience shows that with this class of tool steel a tempering 
heat of 300 degrees renders the metal useless for woodworking. 
The results of the four sets of experiments are given in Table II. 
The values are graphically represented in the engraving, Fig. 1, 
from which it will be seen that the greatest difference occurs 



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TEMPERATURE, DEGREES C. 


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Fig. 1. Diagram showing Percentage of Loss of Hardness in Hardened 
Steel, when Tempered to Various Temperatures, as measured by 
Different Hardness Testing Methods 

at about 300 degrees C, the loss of quenching hardness due to 
tempering being about four times as great when tested by the 
two first methods as compared with the results obtained when 
the steel is tested by the two latter methods given in the table. 
Martens and Heyn have recently pointed out that in the Brinell 
ball test for hardness, the indentations are frequently not cir- 



HARDNESS TESTING 245 

cular, and are therefore difficult to measure, and that when 
testing hard materials the ball itself is appreciably flattened 
while under load. To diminish these sources of error Martens 
has introduced a special form of apparatus for measuring the 
depth of the indentation. 

Relation between Hardness and Wear of Steel. — In a paper 
presented before the International Association for Testing 
Materials, at its congress in New York, September, 191 2, Mr. 
E. H. Saniter stated, as the results of experiments made, that 
there is no definite relation between hardness, as measured by 
the Brinell hardness testing method, and wear. While, in gen- 
eral, a high Brinell hardness number may be expected to indicate 
a metal which will give better wear, there are so many exceptions 
that this test for indicating wearing properties would be unreli- 
able. As an example, it was mentioned that Hadfield's man- 
ganese steel, which has a low Brinell hardness number, proved 
the best steel as far as wear is concerned in the wear tests under- 
taken. However, the relation of either Brinell tests or ordinary 
wear tests to wear in actual practice is a subject which requires 
further investigation, and it is not certain that the ordinary 
type of wear test is a fully reliable indication of the wear a cer- 
tain material may give when in use under certain conditions. It 
would seem, from opinions expressed at the congress for testing 
materials, that wear tests should be made along different lines 
according to the actual uses to which the metal is to be put. 

The Brinell Method of Testing the Hardness of Metals. — 
The method of testing the hardness of metals devised by 
Mr. J. A. Brinell has received very favorable attention from 
metallurgists in this, as well as in other countries. In 1900 
Mr. Brinell, then chief engineer and technical manager of the 
Fagersta Iron and Steel Works in Sweden, first made public his 
method of testing the hardness of iron and steel, by submitting 
it to the Society of Swedish Engineers in Stockholm. At the 
meeting of the International Congress for Testing Materials in 
Paris the same year the method attracted general attention, and 
its merits were duly acknowledged by awarding the inventor 
with a personal Grand Prix at the Paris Exposition. 



246 HEAT-TREATMENT OF STEEL 

The method was first described in the English language by Mr. 
Axel Wahlberg in a paper before the Iron and Steel Institute 
in 1 901. Since then, the practical value of this method has 
been amply substantiated on various occasions by means of 
comprehensive tests and investigations undertaken by several 
distinguished scientists in different countries. In working out 
his method, Brinell kept in view the necessity of taking into 
account the requirements that the method must be trustworthy, 
must be easy to learn and apply, and capable of being used on 
almost any piece of metal, and particularly, to be used on metal 
without in any way being destructive to the sample. 

Principle of Brinell Method for Testing Hardness of Metals. 
— The Brinell method, as already mentioned, consists in partly 
forcing a hardened steel ball into the sample to be tested 
so as to effect a slight spherical impression, the dimensions of 
which will then serve as a basis for ascertaining the hardness 
of the metal. The diameter of the impression is measured, and 
the spherical area of the concavity calculated. On dividing the 
amount of pressure required in kilograms for effecting the im- 
pression, by the area of the impression in square millimeters, an 
expression for the hardness of the material tested is obtained, 
this expression or number being called the hardness numeral. 

In order to render the results thus obtained by different tests 
directly comparable with one another, there has been adopted 
a common standard with regard to the size of ball as well as to 
the amount of loading. The standard diameter of the ball is 
10 millimeters (0.3937 m ch) and the pressure 3000 kilograms 
(6614 pounds) in the case of iron and steel, while in the case of 
softer metals a pressure of 500 kilograms (1102 pounds) is used. 
Any variation either in the size of the ball or the amount of 
loading will be apt to occasion more or less confusion without 
there being any advantage to compensate for such inconven- 
ience. Besides, making any comparisons between results thus 
obtained in a different manner would be more or less trouble- 
some, and complicated calculations would be required. 

The diameter of the impression is measured by means of a 
microscope of suitable construction, and the hardness numeral 



HARDNESS TESTING 247 

may be obtained without calculation directly from the table 
given herewith, worked out for the standard diameter of ball 
and pressures mentioned. The formulas employed in the calcu- 
lation of this table are as follows: 

y = 2 717-0- V> 2 -£ 2 ), ' (1) 

B-f w 

in which formulas 

r = radius of ball in millimeters; 
R = radius of depression in millimeters; 
y = superficial area of depression in square millimeters; 
K = pressure on ball in kilograms; 
H = hardness numeral. 

Suppose, for instance, that the radius of the ball equals 5 milli- 
meters (0.1968 inch), and that the test is undertaken on a piece 
of steel, the pressure consequently applied being 3000 kilograms 
(6614 pounds). Assuming that we found the radius of the de- 
pression equal to 2 millimeters (0.07874 inch) by measurement, 
we have: 

2*" X 5 (5 ~ ^25 - 4) = I3-I3 = y> 
and 

^=228=i7, 

i3-!3 

which as we see agrees with the figure given in our table for 
4 millimeters diameter of impression. 

Relation between Hardness of Materials and Ultimate 
Strength. — It has been pointed out by Mr. Brinell himself 
that this method of testing the hardness of metals offers a most 
ready and convenient means of ascertaining within close limits 
the ultimate strength of iron and steel. This, in fact, is one of 
the most interesting and important results of this method of 
measuring hardness. In order to determine the ultimate strength 
of iron and steel, it is only necessary to establish a constant 
coefficient determined by experiments which serves as a factor 
by which the hardness numerals are multiplied, the product 
being the ultimate strength. Rather comprehensive experiments 
were undertaken with a considerable number of specimens of 



248 



HEAT-TREATMENT OF STEEL 



a 



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Numeral 


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HARDNESS TESTING 



249 



annealed material obtained from various steel works, for the 
purpose of establishing the coefficients, by the present direc- 
tor of the Office for Testing Materials of the Royal Technical 
Institution at Stockholm. The results obtained were as follows: 
For hardness numerals below 175, when the impression is ef- 
fected transversely to the rolling direction, the coefficient equals 
0.362; when the impression is effected in the rolling direction, 
the coefficient equals 0.354. 



£ 50 
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0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1. 

PERCENTAGE OF CARBON 



0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0 

PERCENTAGE OF CARBON 





































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0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0 

PERCENTAGE OF CARBON 



0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0 

PERCENTAGE OF CARBON 

Machinery 



Fig. 



Diagram showing Relation between Results obtained by Various 
Methods of Ascertaining the Ultimate Strength of Materials 



For hardness numerals above 175, when the impression is 
effected transversely to the rolling direction, the coefficient 
equals 0.344; when the impression is effected in the rolling direc- 
tion, the coefficient equals 0.324. 

If the hardness numerals are multiplied by these coefficients, 
the result obtained will be the ultimate tensile strength of the 
material in kilograms per square millimeter. It is evident that 
coefficients can easily be worked out so that if the hardness 
numerals be multiplied by these the strength could be obtained 
in pounds per square inch. 



250 HEAT-TREATMENT OF STEEL 

Suppose, for instance, that a test of annealed steel by means 
of the Brinell ball test gave an impression of a diameter of 
4.6 millimeters. Then the hardness numeral, according to our 
table, would be 170, and the ultimate tensile strength conse- 
quently 0.362 X 170 = 61.5 kilograms per square millimeter, 
provided the impression was effected transversely to the rolling 
direction. 

In Fig. 2 are shown a number of diagrams which indicate the 
results obtained from the tests undertaken to ascertain the co- 
efficients given. In these diagrams the full heavy line indicates 
the tensile strength of the material, as calculated from the ball 
tests in the rolling direction. The dotted lines indicate the 
strength as calculated from the ball tests in a transverse direc- 
tion, and the " dash-dotted " lines show the actual tensile 
strength of the material as ascertained by ordinary methods for 
determining this value. It is interesting to note how closely the 
three curves agree with one another, and considering the gen- 
eral uncertainty and variation met with when testing the, same 
kind of material for tensile strength by the ordinary methods, 
it is safe to say that the ball test method comes nearly as close 
to the actual results as does any other method used. Especially 
within the range of the lower rates of carbon, or up to 0.5 per 
cent, or in other words, within the range of all ordinary con- 
struction materials, the coincidents are, in fact, so very nearly 
perfect as to be amply sufficient to satisfy all practical require- 
ments. 

In the case of any steel, whether it be annealed or not, that has 
been submitted to some further treatment of any other kind 
than annealing, such as cold working, etc., or in the case of any 
special steel, there would be other coefficients needed which 
would then also be ascertained by experiments. The same co- 
efficient, however, will hold true for the same kind of material 
having been subjected to the same treatment. Thus, the ball 
testing method for strength is equally satisfactory, and far more 
convenient, in all cases where the rupture test would be applied. 
One of the greatest advantages of the Brinell method is that 
in the case of a large number of objects being required to be 



HARDNESS TESTING 25 1 

tested, each one of the objects can be tested without demolition, 
and without the trouble of preparing test bars. 

Practical Constants for Relation between Hardness and 
Strength. — In a paper entitled " Researches on the Hardness 
of Steel," read by Capt. C. Grard before the Congress of the 
International Association for Testing Materials, held in New 
York City, September, 191 2, the relation between hardness and 
tensile strength, as determined in recent investigations, was ex- 
haustively dealt with, and the following coefficients were given 
for different grades of steel: 

Steels, extra soft K = o. 360 

Steels, soft and semi-hard K = o. 355 

Steels, semi-hard K = o. 353 

Steels, hard K = o . 349 

It will be seen that these coefficients differ by but a slight 
amount and it was suggested by another member of the congress 
of testing materials that a uniform constant be adopted by the 
International Association for Testing Materials, which would 
be used for calculating the tensile strength directly from the 
results of the hardness tests. 

Accuracy of Brinell Test. — In a paper read before the Con- 
gress of the International Association for Testing Materials 
held in New York City, September, 191 2, an investigation into 
the accuracy possible with the Brinell hardness testing method 
was recorded. From this investigation it appears that when 
commercial apparatus, as ordinarily used for making the Brinell 
test, is employed, and the test is carried out with ordinary care 
and precaution, it is reliable within an error of five Brinell units 
above or below the actual hardness. In other words, if the 
hardness of two pieces of metal is tested, and the difference on 
the Brinell scale is more than ten hardness units, it is certain 
that there is an absolute difference in the hardness of the pieces 
tested. With regard to the conditions under which the tests 
should be made, it may be stated that the pressure should be 
gradually applied during a time of two minutes or more, and 
the pressure should be kept on the test piece for a period of at 
least from two to five minutes. 



252 



HEAT-TREATMENT OF STEEL 



The Time Element in Hardness Tests by the Brinell System. 

— A diagram indicating the effect of the time element in hard- 
ness tests made by the Brinell method is shown in Fig. 3. The 
tests upon which this diagram is based were made by the German 
Glyco Metal Co. On the lower scale is given the time, in min- 
utes, during which the pressure on the metal was permitted to 
act, while the scale on the left-hand side gives the hardness 




0.5 1.0 



2.0 

MINUTES 



4.0 5.0 



Machinery 



Fig. 3. 



Curves showing the Variation in Results obtained by Hardness 
Tests of Varying Duration 



numerals according to the Brinell hardness scale. It will be 
noted that the longer the pressure was permitted to act, the 
greater was the impression made on the metal, so that a lower 
hardness numeral resulted. Two sets of curves are shown, one 
with the metal heated to 176 degrees F., and one with the metal 
at 68 degrees F. It is interesting to note that the curves in each 
set are almost parallel, except in one case, thus indicating that 



HARDNESS TESTING 253 

for comparative purposes the Brinell test is accurate no matter 
what the length of duration of pressure, provided, of course, 
that the various samples tested are all subjected to pressure 
for the same length of time. 

It is also interesting to note the difference in the hardness of 
the metal brought about by the change in temperature. It 
will be seen that at the higher temperature its hardness numeral 
is not only less than at the lower temperature, as, of course, 
would be expected, but when the pressure on the metal is per- 
mitted to remain for a longer time, the metal apparently gives 
way much easier to continuous pressure when heated than when 
at a lower temperature. Tests of this kind should be of great 
value in determining the relative value of bearing metals which 
for long periods are to be subjected to heavy pressures under 
increasing temperatures. A new factor in hardness testing is 
also introduced, which the Brinell method is particularly adapted 
to measure, viz., the power of resistance of various metals to 
continuous pressure, a factor which may be found to vary con- 
siderably for different materials. 

Application of the Brinell Ball Test Method. — Summarizing 
what has been said in the previous discussion, and adding some 
other important points, we may state the various uses for which 
the Brinell ball test method may be applied, outside of the 
direct test of the hardness of construction materials and the 
calculation from this test of the ultimate strength of the mate- 
rials, as follows: 

1. Determining the carbon content in iron and steel. 

2. Examining various manufactured goods and objects, such 
as rails, tires, projectiles, armor plates, guns, gun barrels, struc- 
tural materials, etc., without damage to the object tested. 

3. Ascertaining the quality of the material in finished pieces 
and fragments of machinery, even in such cases when no speci- 
men bars are obtainable for undertaking ordinary tensile tests. 

4. Ascertaining the effects of annealing and hardening of steel. 

5. Ascertaining the homogeneity of hardening in any manu- 
factured articles of hardened steel. 

6. Ascertaining the hardening power of various quenching 



254 HEAT-TREATMENT OF STEEL 

liquids and the influence of temperature of such liquids on the 
hardening results. 

7. Ascertaining the effect of cold working on various materials. 

Machines for Testing the Hardness of Metals by the Brinell 
Method. — The method of applying the Brinell ball test was 
at first only possible in those establishments where a tensile 
testing machine was installed. As these machines are rather 
expensive, the use of the ball test method was limited. For 
this reason a Swedish firm, Aktiebolaget Alpha, Stockholm, 
Sweden, has designed and placed on the market a compact 
machine specially intended for making hardness tests. This 
machine, the most important working mechanism of which is 
shonw in Fig. 4, consists of a hydraulic press acting downward, 
the lower part of the piston being fitted with a 10-millimeter 
steel ball k by means of which the impression is to be effected 
in the surface of the specimen or object to be tested. This 
object is placed on a support (not shown) which is vertically 
adjustable by means of a hand-wheel, while at the same time 
it can be inclined sideways when this is needed on account of 
the irregular shape of the part tested. The whole apparatus 
is solidly mounted on a cast-iron stand. The pressure is effected 
by means of a small hand pump, and the amount of pressure 
can be read off directly in kilograms on the pressure gage mounted 
at the top of the machine. 

In order to insure against any eventual non-working of the 
manometer, this machine is fitted with a special contrivance 
purporting to control in a most infallible manner the indications 
of that apparatus, while at the same time serving to prevent 
any excess of pressure beyond the exact amount needed according 
to the case. This controlling apparatus consists of a smaller 
cylinder, a, directly communicating with the press-cylinder. 
On being loaded with weights corresponding to the amount of 
pressure required, the piston in this cylinder will be pushed up- 
ward by the pressure effected within the press-cylinder at the 
very moment when the requisite testing pressure is attained. 
Owing to this additional device, there can thus be no question 
whatever of any mistake or any errors as to the testing results, 



HARDNESS TESTING 



255 



that might eventually be due to the manometer getting out of 
order. 

Method of Performing the Ball Test. — The test specimen 
must be perfectly plane on the very spot where the impression 
is to be made. It is then placed on the support, which, as men- 




Fig. 4- 



Section of Working Mechanism of a Hardness Testing Machine 
employing the Brinell Principle 



tioned, is adjusted by means of a hand-wheel so as to come into 
contact with the ball k. A few slow strokes of the hand pump 
will then cause the pressure needed to force the ball downward, 
and a slight impression will be obtained in the object tested, 
but as soon as the requisite amount of pressure has been attained, 



256 HEAT-TREATMENT OF STEEL 

the upper piston is pushed with the controlling apparatus up- 
ward, as previously described. On testing specimens of iron 
and steel, the pressure is maintained on the specimen for 2 min- 
utes, but in the case of softer materials for at least 5 minutes. 
After the elapse of this time, the pressure is released, and the 
contact between the ball and the sample will cease. A spiral 
spring fitted within the cylinder, and being just of sufficient 
strength to overcome the weight of the press piston, pulls the 
same upward into its former position, while forcing the liquid 
back into its cistern. The diameter of the impression effected 
by the ball is then measured by a microscope, which is specially 
constructed for this purpose, the results obtained by this meas- 
urement being exact within 0.05 millimeter (0.002 inch). 

Another type of machine is designed for special tests in which 
very high pressures are required. The ball in this machine is 
19 millimeters (0.748 inch) in diameter, and the pressures em- 
ployed vary from 3 to 50 tons. The construction and operation 
of the larger machine are otherwise exactly the same as that of 
the machine in Fig. 4. 

Derihon Portable Form of Brinell Hardness Testing Machine. 
— The only disadvantage of the Brinell method for general 
practical use lies in the apparatus required and the comparative 
slowness of the operation. The apparatus just described, con- 
structed in the form of a small hydraulic press, operated by 
hand, is rather heavy. It is evident that with such an apparatus, 
the work has to be brought to the machine, so that its regular 
use for inspection purposes in different parts of a manufacturing 
plant is impracticable. 

The foregoing considerations lend interest to the apparatus 
shown in Fig. 5, which was devised by the Derihon Steel 
Works of Loucin-lez-Liege, Belgium. This firm is an impor- 
tant manufacturer of high-grade drop forgings for automobile 
work, and it originally developed the machine for use in its own 
plant. As may be seen, the pressure is applied by a hand-oper- 
ated screw, and the press is small enough to be perfectly portable, 
weighing only about 12 pounds. 

The sectional view in Fig. 5 shows clearly the action of the 



HARDNESS TESTING 



257 



apparatus. The work is placed on the platen F, which rests in 
a spherical seat on top of adjusting screw G. By means of this 
self-adjusting seat, the work gets an even bearing and a direct 
pressure, even though its under surface may be quite out of 



-» r «r-LEAD=8 m / n 



D 




I |-») **-LEAD=7.25 m /m 
r'-LEAD =8 m / m 




LEAD = 7.25 m / m 




Fig. 5. 



Section of the Derihon Hardness Testing Apparatus employing 
the Brinell Principle 



true. The purpose of the adjusting screw G is, of course, to give 
a rapid adjustment for the thickness of the work. It will take 
in about 3 inches as shown. The thread of G, while of coarse 
pitch, still lies within the angle of repose, so that it is not dis- 
turbed when the pressure is applied by levers M. 



258 HEAT-TREATMENT OF STEEL 

A differential screw mechanism is used for applying the pres- 
sure. This mechanism consists of the double handle M, keyed 
to the sleeve D, which is threaded into the stationary nut E, 
and onto the ram C; this latter is kept from revolving by a stop 
K, which enters a slot cut in the flange, and is provided with 
a threaded chuck for holding the ball B. Sleeve D is, it will 
be seen, the only revolving member of this differential screw. 
The thread on the inside has a lead of 8 millimeters, while that 
on the outside has a lead of 7.25 millimeters. This gives an 
advance of 8 — 7.25 = 0.75 millimeters, or about 0.03 inch per 
revolution of the levers M. The advantage of this construction 
is, of course, that it gives the effect of a fine thread with a lead 0.03 
inch, with the use of threads coarse enough to withstand the great 
strain to which they are subjected in tightening down the work. 

The most ingenious feature of the mechanism is the provision 
made for gaging the pressure with which the ball is forced into 
the work. By the arrangement used, the frame A of the press 
itself serves as the spring by which the pressure is measured. 
As the ball is forced into the work with greater and greater pres- 
sure, the frame A is deflected and B and F spring apart. Arm 
H is screwed to A at its lower end as shown, but is free at its 
upper end. Here it is provided with a bearing point X close 
to fulcrum Y of pointer P. As the frame A springs under the 
pressure, H, being free at the upper end, remains undistorted 
and stationary, while pivot Y rises. As pivot Y rises, lever P 
swings downward, since it rests only on the point X of stationary 
arm H. The lower end of lever P is provided with a thin metal 
disk R, which is thus swung in an arc of a circle about center 
F. A series of holes Z, bored in the side of the frame, permit 
the position of this disk to be seen. These holes are so calibrated 
that each reads to a definite number of kilograms of pressure 
when disk R is centered with it. Under the extreme tension, 
when central with the left-hand hole, the reading shows the appli- 
cation of a pressure of 3000 kilograms or 6614 pounds. 

The construction of the pivot joint at Y is interesting, and is 
best shown at the right of the engraving in Fig. 5. The hub 
of pointer P is clamped to Y by two set-screws 0, which set in 



HARDNESS TESTING 259 

a V-slot cut in F. Two caps T are screwed on at either side to 
protect the bearings of pivot F. Set-screws V are adjusted to 
take up the end motion of F; they do not, however, restrain 
it in any direction other than the longitudinal. The real bear- 
ing is furnished by the points of screws U (one at each end) in 
the bottoms of the V-grooves. These furnish a knife edge or, 
rather, point support, which gives the utmost freedom and sen- 
sitiveness of movement to the pointer P and the indicating disk 
R. The various adjustments in connection with this bearing, 
and the various contact points in the lever system, will be clearly 
understood from the engraving. 

The simplicity of the operation of this device will be immedi- 
ately appreciated. The object to be tested is placed on the 
platen, the adjusting screw is run up until the work makes con- 
tact with the ball, and then the handles are revolved until the 
indicating disk is centered with the particular hole in the frame 
which shows that the standard pressure has been reached. 
Handles M are then screwed back again, the work is removed, 
and the diameter of the impression in millimeters measured. 
This gives the hardness number directly. The whole operation 
is evidently one of seconds only. 

The Ballentine Hardness Testing Device. — In the Brinell 
hardness testing method the measuring of the dimensions of the 
indentation is more or less difficult to accomplish accurately, 
and the method requires special instruments for obtaining the 
indentation, and for measuring the amount of depression in the 
metal tested. In order to overcome this difficulty, a means 
known as the Ballentine method and apparatus for quickly 
and accurately determining the resistance to indentation of a 
material has been devised and constructed. 

The method employed in the Ballentine device consists in 
allowing a hammer of specified weight to fall through a specified 
height on an anvil to which is connected a pin which rests on 
the specimen to be tested. An indentation in the material is 
obtained, but the resistance encountered, instead of the dimen- 
sions of the indentation, is measured. This resistance is meas- 
ured by the blow of the hammer being transmitted to the test 



260 



HEAT-TREATMENT OF STEEL 



pin through a soft metal recording disk located at the lower end 
of the hammer. This disk affords a constant resistance to 
deformation, and will be indented to a depth varying in propor- 
tion to the resistance the pin encounters in indenting the material 
tested. The recording disk is usually made from lead. 

Fig. 7 shows a sectional view of the apparatus, which consists 
of a guide tube encasing the drop hammer which at the lower 
end is provided with a small anvil to which is clamped a lead 
disk. At the upper end the hammer is held at the top of the 
tube by a spring latch. At the lower end of the tube a test 
pin holder is located, in which are inserted the test pins for 
testing the various materials. The upper end of the test pin 
holder is provided with an anvil of the same diameter as the one 
on the lower end of the hammer. A small spirit level is inserted 




Fig. 6. 



Lead Recording Disk used in the Ballentine Device, before 
and after Test 



in the top of the tube for leveling the apparatus, and two small 
slots are cut in the guide tube for inserting and removing the 
recording disks. The apparatus can be used to test all materials 
which can be ordinarily machined by steel cutting tools, but 
cannot be used for hardened steel and similar materials which 
are too hard to be indented in this manner. Two test pins are 
provided, one for soft materials such as lead and babbitt metals, 
and another for harder materials such as iron and steel. The 
pin for hard materials is very short and small in diameter, while 
the pin for soft materials is longer and larger in diameter. 

The testing can be made either on small test specimens or 
directly on large parts in process of manufacture, the great ad- 
vantage of this hardness tester being that it is entirely self-con- 
tained and well adapted for either laboratory or general shop use. 
To make a test it is only necessary to smooth off a surface on the 
specimen to be tested, and clamp it firmly to some rigid body. 



HARDNESS TESTING 26 1 

In Fig. 6 is shown a lead recording disk before and after the 
test. These disks are made within 0.0015 inch of nominal size 
from a material as nearly of uniform density and hardness as 
obtainable. The disk is measured with a micrometer before 
being placed on the drop hammer. When the test has been 
made, the thickness of the metal between the two recording anvils 
is again measured, and the difference between the two dimensions 
will indicate the resistance to indentation or the hardness of the 
material tested. If, for instance, the disk measured 0.300 inch 
before the test, and 0.156 inch after test, the difference, 0.144 
inch, indicates the hardness of the material, and this hardness 
would be known as No. 144. 

Principle of the Shore Scleroscope. — The Shore scleroscope 
is an instrument in a measure dependent on sensitive touch; 
or, in other words, it feels the substance much the same as the 
human fingers. When we touch two or more objects, as, for 
instance, an orange and an apple, we know that the orange is 
softer because it yields under pressure more than the apple. 
We are powerless to measure the hardness of any object that is 
harder than the finger tips, and there is no way of telling how 
hard it may be by finger pressure alone. 

The sensitive touch of the scleroscope is produced by a tiny 
hammer dropping from a height of about ten inches onto the 
metal, hardened steel, etc., which it penetrates slightly. The 
hammer moves freely, yet snugly, within a glass tube, and 
weighs about 40 grains. Its striking point consists of an in- 
serted diamond of rare cleavage formation, annealed sufficiently 
to withstand shocks. This jeweled point is slightly convex and 
has an area of from about 0.010 to 0.025 square inch. When 
the plunger strikes the metal to be tested, it reacts or rebounds. 
The height of this rebound is read on a graduated scale, and an 
accurate determination of the quantitative hardness of the piece 
under test is thus obtained. 

In the first experiments a steel ball was used as the hammer, 
but the results were only partially satisfactory. In fact, the 
inventor was well-nigh on the point of giving up the method when 
he met the French expert on metals, Dr. Herault. Following 



262 



HEAT-TREATMENT OF STEEL 



f T 




() 



CD 





Machinery 



ig. 7. Ballentine Fig. 8. View showing the Construction of the Shore 
Testing Device Hardness Testing Device known as the Scleroscope 



HARDNESS TESTING 263 

out certain of his suggestions, the inventor succeeded in pro- 
ducing a satisfactory instrument for the testing of hardness. 
The difficulty with the ball-shaped hammer was that it was 
incapable of striking a sufficiently hard blow to get adequate 
results, especially with hardened tool steel, so the area of contact 
was reduced, although the weight was kept large in comparison. 

Hardness vs. Elasticity. — When the hammer of the sclero- 
scope is allowed to drop with no other force than its own weight, 
and the point is so flat that absolutely no impression is made on 
the surface of very hard steel, then the rebound will be about 
90 per cent of the fall. This phenomenon is known as the 
elasticity of solid bodies. Now, since hardness is resistance to 
penetration, in its clearest definition, it stands to reason that 
the point of the hammer must be somewhat reduced and rounded. 
Therefore the relation between the weight of the hammer and 
its point should be such that when it drops on hardened steel, 
a permanent impression must always be made, so that if we 
had not the rebound to go by, the microscope would still show 
the values. When the area of the hammer is thus reduced enough 
to make a permanent impression, a certain amount of the energy 
stored in the hammer is utilized in doing work. This over- 
comes the tendency of the metal to resist penetration, depending 
on how hard it is, or the resistance it offers, and naturally it must 
rebound considerably less. The hammer always delivers a blow 
of exactly the same force. If now we get a rebound of 75 per 
cent on very hard steel, we know that 15 per cent of the hammer's 
energy was spent in its efforts to overcome the resistance of the 
steel before it had a chance to react and repel the missile. 

Description of the Shore Instrument. — While the absolute 
weight of the entire hammer is little, it is very great relative 
to the striking area. The hammer has a cylindrical body and 
is guided in its fall by a glass tube. Great difficulty has been 
experienced in obtaining tubes with a sufficiently perfect bore. 
There seems to be no commercial method of manufacturing 
such tubes, and the method of " test-and-reject " is therefore 
employed, resulting in a very great amount of waste. 

The glass tube is secured to a frame in a vertical position with 



264 HEAT-TREATMENT OF STEEL 

the lower end open. The operation of the instrument is very 
simple. When the hammer is to be raised to the top, the bulb 
A, Fig. 8, is pressed and then suddenly released. This sucks up 
the jeweled plunger hammer referred to so that it may be caught 
by a hook which is suspended exactly central in the glass tube 
and engages with an internal groove on the top of the hammer. 
Adjusting screws B for the hook and its spring are contained in 
the removable knurled cap. C is a cylinder and piston for re- 
leasing the hook and hammer by bulb H whenever a test is to 
be made; / is a hook which is pressed at the same time and 
which opens a valve letting in the air and thus preventing the 
occurrence of a vacuum when the hammer drops. At J is 
shown a pinion knob for moving the instrument up and down 
independently of the heavy rack and clamp F actuated by the 
lever G. At E are shown leveling screws and at D a plum rod. 

Application of the Shore Instrument. — When small pieces 
are to be tested, the scleroscope, as shown in Fig. 8, self-contained 
with its clamp and anvil, is employed. In using the instrument 
with the stand the specimen is placed on the table or secured in 
a holder. It is necessary that the actual point tested should be 
clean and horizontal and that the piece should be firmly held. 
If necessary to test more than once, the piece should be slightly 
moved so as to expose a fresh point to the hammer. The inden- 
tation made is, however, very minute, so that several are usually 
unobjectionable. 

When the ends of rods, drills, and many other tools are to be 
tested, they are clamped in a bench vise, and a swinging arm is 
employed; the instrument is removed from its post on the clamp 
frame by knurled set-screw K, and is attached in the same way 
to the post on the swinging arm. A kind of female dove- tailed 
finger ring attached to the clamp on the dove-tail rack bar of 
the instrument is provided for use in free-hand testing on very 
large floor work, on parts of machinery being assembled, or on 
the stock rack, etc. From what has been said, it is apparent 
that the apparatus is of universal application. 

The Shore Hardness Scale. — Instead of dividing the whole 
length of the fall of the hammer into a scale consisting of 100 



HARDNESS TESTING 265 

divisions, the figure 100 is carried down to a point representing 
about 68 per cent of the total height of the scale as shown in 
Fig. 8. This was not an arbitrary provision, but was adopted 
after consultation with leading metallurgists, one of whom was 
Dr. Paul Herault, of aluminum and electric steel making fame, 
of France. These authorities agreed that in the scleroscope 
hardened steel of average hardness should be taken as the stand- 
ard with which all other less hard metals should be compared; 
100 is the average hardness of hardened steel; 90 is a low value, 
while no is a high value. 

The scale, therefore, makes it an easy matter to compare the 
various metals, no matter what their hardness is, and the re- 
bound of the hammer is, therefore, measured against a scale 
graduated from o to 140. This scale is secured in position back 
of the glass tube. To aid in reading the rebound, a magnifying 
glass is supplied. After some practice the assistance of this 
glass may be dispensed with. However, when used, it is secured 
in such a position as to cover the probable region of the expected 
reading. The rod to the left of the tube is the support to which 
the magnifying attachment is secured and along which it is ad- 
justed. The rod to the right of the tube is a plum rod; it swings 
freely from a point of attachment above, and enables the oper- 
ator to keep the glass tube vertical. 

With the scale graduated from o to 140, with hardened steel 
at or near 100, the hardness of all ordinary materials can be 
measured. Porcelain and glass, of course, have a higher hard- 
ness number than hardened steel, while unhardened steels, brass, 
zinc and lead have gradually lower degrees of hardness. Un- 
hammered or unrolled lead produces a rebound of only two 
graduations. 

Some Uses for the Scleroscope. — One of the results of the 
introduction of scientific methods of precise quantitative meas- 
urement of hardness promises to be in the determination of the 
relation of the cutting tool to the work to be machined. We are 
all aware that the tool must be harder; but how much harder? 
And how express this relation in intelligible language? The 
scleroscope, it is hoped, will afford a fairly definite answer to 



266 HEAT-TREATMENT OF STEEL 

this problem. The law has been laid down that the comparative 
hardness between tool and work, as determined by scleroscope 
readings, should be in the ratio of 3 to 1 or 4 to 1, in order to 
secure the best commercial results. 

For example, take the case of work to be machined consisting 
of a 1 per cent carbon tool steel. Unannealed, such steel is 
found upon testing to have a hardness varying from 40 to 45 
points. According to the above law, the cutting tool should 
be at least about 120 to 135 points hard; but the same steel, 
properly annealed, is only about 31 points hard. Consequently, 
it is not difficult to find a suitable material for the cutting tools. 
A good quality of carbon tool steel, well hardened, has a hardness 
of from 95 to no, and is consequently suitable to cut material 
of a hardness of 31. Now if this principle as to relative hard- 
ness can be thoroughly established for all kinds of metals, an 
element of certainty will be introduced into shop practice. 

Again, it is, of course, to be expected that if two metal parts 
wear or rub against each other, the harder of the two will cut the 
softer, whether the difference is small or great, so that it is often 
important to know whether the more expensive part is really 
the harder. The scleroscope would seem to afford a means of 
determining with precision slight differences in hardness, thus 
enabling the manufacturer to assemble contacting moving parts 
on the principle of a harder expensive piece in association with a 
softer cheaper one. Thus in an electrical repair shop, instances 
may readily be found of the steel shaft cut by the brass box, 
the box cut by the shaft, and a fairly even wear of both. From 
an economical point of view, it is much better to have the brasses 
worn than the shaft, and with such an instrument as the sclero- 
scope it would be possible to predetermine this economically 
better result. It would seem an easy matter for an automobile 
manufacturer, say, so to specify the hardness of the gears used, 
that the gear manufacturer could supply him with a uniform 
product. 

An important application of quantitative hardness tests would 
appear to be in connection with high-speed steels. Such steels 
disclose, upon testing with this instrument, a hardness varying 



HARDNESS TESTING 267 

from 80 to 105. This is at ordinary temperatures, however, 
and shows scarcely as high a degree of hardness as the best of 
the pure carbon steels. The effectiveness of high-speed steels 
depends largely upon the fact that at temperatures of from 600 
to 1000 degrees F., at which carbon steels would lose their 
temper, they retain a high degree of hardness, amounting, say, 
to 75 on the scleroscope scale. This is sufficient — following 
the principle of 3 to 1 — to do heavy machining on annealed 
machine steel having a hardness of 25 on the same scale. But 
if the heat developed by high speed and heavy cuts succeeds in 
lowering the hardness of the high-speed steel of the tool much 
lower than 75, then it is no longer an effective tool. It becomes 
of importance then to test high-speed steels for their effective- 
ness under temperature conditions obtaining in actual service. 
It is a comparatively unimportant matter to know that a cer- 
tain tool of high-speed steel is very hard when cold; what is its 
condition when hot? By heating the tool to the required tem- 
perature, and then testing with the scleroscope, this condition 
may be determined. Thus the real effectiveness of the high- 
speed steels may be determined in advance of their use, or even 
of their purchase. 

Amount of Pressure for Indentation. — When the hammer 
falls through a height of ten inches onto hardened steel, it will 
deliver a striking energy equal to about 20,000 times its own 
weight, acting through a very short space, of course. With a 
hammer weighing about 40 grains, and an indentation of, say, 
0.002 inch depth, a working pressure of about 100 pounds is 
obtained. This force, acting on a convex point of ^ inch 
diameter, is concentrated. The pressure thus available is about 
500,000 pounds per square inch, which is ample to exceed the 
elastic limit of the hardest and strongest steel in existence. 

A remarkable feature of this instrument is that it is self-com- 
pensating with regard to the energy of the hammer blows on 
the softer metals. This is due to the yielding of the material 
and the comparatively slow stoppage of the hammer. In lead, 
for example, a deep impression is made. This requires a great 
amount of energy, which is nearly all spent in doing work, and 



268 HEAT-TREATMENT OF STEEL 

there is very little rebound afterward — about 3 degrees as 
against no for the hardest steel. The constant pressure de- 
veloped by the hammer is thus only 12 pounds instead of 100 or 
more for good hard steel, and, of course, the pressures for inter- 
mediate hardnesses as on brass and soft or tempered steel are 
always in proportion to the physical hardness of the brass or 
steel. 

Application to Shop Work. — The manufacturer who wishes 
to get high efficiencies out of his tools will not benefit by the 
help of such a commodity as the scleroscope in detecting good 
and bad tools, unless he is willing to amend the errors in practice 
which he may find. The observation of this principle is the 
foundation of the success which hundreds of firms in this country 
and Europe are having with this instrument. While tool work 
is a line requiring the most careful attention, the material worked 
and produced is none the less important. In this connection 
the scleroscope is very commonly applied to industrial systems, 
with admirable results. An instance may thus be cited showing 
how these results are obtained. 

In 1908, the Brown & Sharpe Mfg. Co. adopted the new method 
as a guide in the laboratory, particularly for the study and selec- 
tion of such steel as is required in standard commercial tools. 
The attention of the company was then turned to its high-grade 
automobile gears of alloy steels, etc. Meanwhile the Packard 
Motor Car Co. used the scleroscope to study the past perform- 
ances of the various gears and parts of old Packard cars, and 
made careful records. This was also done by many other con- 
cerns, and these records showed that alloys, steel or non-ferrous 
metals would give a certain efficiency if the hardness was just 
right. As the best is, in the end, the cheapest, in high-grade 
apparatus, the Packard engineers began to issue orders to their 
various auto part making houses for material which was specified 
to require a given degree of scleroscope hardness. Gears were 
made for them by the Brown & Sharpe Mfg. Co. and the Gleason 
Works, both of whom are using the scleroscope to aid them in 
filling orders. 

Wyman & Gordon, who supply forgings to Brown & Sharpe, 



HARDNESS TESTING 



269 



were able to make them to the required specifications, but, in 
order to do so, they had to see that the raw material was of the 
proper hardness. This brings the matter back to the open- 
hearth or crucible and chemical laboratory, where again the 
scleroscope is used to great advantage. Before the completion 
of an automobile of the guaranteed kind, often a dozen instru- 

Table IV. Scleroscope Hardness Scale* 



Name of Metal 



Lead (cast) 

Babbitt metal 

Gold 

Silver 

Brass (cast) 

Pure tin (cast) 

Brass (drawn) 

Bismuth (cast) 

Platinum 

Copper (cast) 

Zinc (cast) 

Iron, pure 

Mild steel, 0.15 per cent carbon 

Nickel anode (cast) 

Iron, gray (cast) 

Iron, gray (chilled) 

Steel, tool, 1 per cent carbon. . . 
Steel, tool, 1.65 per cent carbon 

Vanadium steel 

Chrome-nickel 

Chrome-nickel (hardened) 

Steel, high-speed (hardened) . . . 
Steel, carbon tool (hardened) . . . 



Annealed 



2-5 
4-9 

5 
6H 

7-35 
8 
10-15 

9 
10 

6 

8 
18 
22 
3i 
3o-45 

30-35 
35-40 
35-45 
47 



Hammered 



3-7 

20-30 



24-25 

17 

14-20 

20 

25-30 

3o-45 

55 

50-90 
40-50 



60-95 

70-105 

70-105 



* The figures given are subject to variation, owing to the differences in composition of the 
metals tested. 

ments are used among the specialty makers who supply the 
various parts. The ball-bearing manufacturers are required 
and prefer to test every part before assembling. The Hyatt 
Roller Bearing Co. and the Hess-Bright Mfg. Co. are obliged 
to use a number of scleroscopes which are operated by women, 
carefully trained, who are able to pass on a large number of pieces 
daily. This testing is to ascertain principally two factors on 
which success in service depends, viz. : the right degree of hard- 
ness, and the uniformity of this hardness — and both are equally 
important. In the latter case it is necessary to test the parts 



270 



HEAT-TREATMENT OF STEEL 



in a number of places, which must be done very rapidly to keep 
down the additional cost, particularly as in the manufacture of 
standard parts such as these, there are always losses due to the 
rejection of some parts which do not conform to the specifica- 
tions. 

The Lunkenheimer Co., the Light Mfg. & Foundry Co., and 
other up-to-date manufacturers, use the scleroscope in the stand- 
ardization of castings adapted to various needs. These houses 



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DEGREES g § 

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Fig. 9. 



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Machinery, N. Y. 

Hardness Curve for Tool Steel of about 0.90 per cent Carbon 



also make auto parts for the Packard Co., etc., and by the use 
of the scleroscope are enabled to live up to their specifications. 
In these auto shops the instrument is used for all classes of 
work, although it is most needed in the inspection department 
for the examination of parts and material, particularly of those 
not made by the builders. 

Tool Steel and the Scleroscope. — Since for most uses (other 
than for turning or planing tools) plain carbon steel is as yet 
adequate, many manufacturers have turned their attention to 
the art of obtaining much higher efficiencies by aid of the scle- 



HARDNESS TESTING 



271 



roscope after good steel has been selected. The method of 
doing this is interesting, and was first hailed as a revelation by 
many authorities. Thus, when a steel having a carbon content 
of 0.90 of one per cent and over is heated to the right temper- 
ature and is then properly quenched, the limit of hardness and 
strength is obtained. Now, attaining this temperature is such 
a delicate matter, that unless the very best facilities are at 
command, anything but the exact heat required may be obtained, 

p 



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DEGREES g §g§gSSS§gS§ 

FAH'T 3£« ^rtSSrtSSSSlN 

Machinery, N. Y. 

Fig. 10. Hardness Curve for Tool Steel of 1.65 per cent Carbon 

and if the heat is too low the tool will be hard only on the edges, 
while if it is only a trifle overheated, such as is regularly done 
by the average hardener who takes chances by depending on 
the skill of the eye, something like from 50 to 75 per cent of the 
strength due to rolling or forging is lost. This appalling loss in 
strength so vitally important in any tool is accompanied by a 
slight drop in the hardness — not more than 5 per cent. 

This is detected by the scleroscope as shown in charts, Figs. 9 
and 10. The former is a hardness curve taken from a tool steel 
of about 0.90 per cent carbon, while the latter is one taken from 



272 HEAT-TREATMENT OF STEEL 

a steel having 1.65 per cent carbon. The difference between 
these curves is indeed very striking and very significant to those 
who have mastered the elementary principles of the study of 
tool steels by aid of the scleroscope. The curves are obtained 
from the Metcalf test as follows: A piece of steel a few inches 
long and about one-half inch square is heated to a bright yellow 
on one end and manipulated so that the temperature is less and 
less toward the other end until a red is scarcely visible. The 
piece is then quenched in water, ground clean, and tested by the 
scleroscope at intervals of about | inch along the bar, beginning 
at the unhardened end. As each section is tested, a reading is 
obtained which corresponds with the exact hardness that would 
be obtained by quenching a similar piece at whatever heat the 
said test piece had in that location. This hardness number is 
plotted out on a chart in the usual way so that a true curve is 
obtained showing the character of the changes in hardness and 
strength. 

The test piece thus obtained represents a stock bar, of the 
steel which is to be worked into tools, dies, etc., hardened at 
temperatures that vary more widely than could occur in any 
well-regulated hardening room, and somewhere within these 
limits is the temperature that yields the maximum hardness. 
It supplies an expedient whereby the hardener may know exactly 
what temperature is most suitable for each tool made from the 
steel thus tested. His future work is guided and facilitated by 
stamping on each tool a number corresponding to the hardness 
number given to the said stock bar. It also enables the hardener 
or the inspector to intelligently test all hardened tools. Thus, 
if a die is hardened to 95, we can determine by referring to the 
test piece of steel, whether this is the highest degree of hardness 
obtainable with this steel. If the test piece showed the hard- 
ness to be, say, 100 or no, and the die only showed 95/ it would 
indicate that the die did not fulfill the necessary requirements. 



INDEX 



All°y steels, casehardening, no, 204 
composition, heat-treatment and 
properties, 93, 104, no 
effect of heat-treatment, 103 
American Gas Furnace Co.'s gas case- 
hardening furnace, 222 
American Society for testing materials, 

methods of casehardening, 201 
Annealing, carbon steel, 79 

effect of previous, on hardness, 19, 20 
high-speed steel, 85, 90 
"water," 89 
Arc heating for local hardening, electric, 

i53 
Automobile Engineers Society, heat- 
treatment methods for carbon and 
alloy steels, 104, no 

Baily electric furnace, 146 

Ballentine hardness testing device, 

259 
Barium-chloride baths for heating steel, 

Si, 54 
disadvantages of, 136 
Barium-chloride electric furnaces, 128, 

140 
Baths, for heating steel, barium-chlo- 
ride, 51, 54, 136 
cyanide of potassium, 50 
lead, 50 
liquid, 49 

metallic salt, for electric furnace, 132, 
136 
Baths, quenching, 61 

quenching, for casehardening, 201 
quenching, for gears, 232 
quenching, for pack-hardening, 166 
quenching, for steel castings, 176 
quenching, list of different kinds, 65 
quenching, oils used for, 65 
quenching, temperature of, 64 



Baths, tempering, 70, 73, 74 

tempering, for gears, 233 
Boxes, for casehardening, 189 

for casehardening gears, 228 
Brayshaw's experiments on heat-treat- 
ment of steel, 15 
Brinell's hardness testing method, 239, 

245 
accuracy of, 251 
applications of, 253 
hardness numerals, 248 
machines used for, 254 
used for determining ultimate 
strength, 247 
Burned steel, restoring, 78 

Calibration of pyrometers, 57, 60, 233 
Carbon, action of, in heated steel, 
121 
Carbon steel, annealing, 79 

composition, heat-treatment and 

properties, 104, no 
experiments on heat-treatment of, 15 
hardening, 1 

properties and use of, 95, 104 
tempers for tools made from, 76 
versus high-speed steel, 23 
Carbonaceous gas used for caseharden- 
ing, 211 
Carbonizing process, 195 
Casehardened parts, steel for, 96, 181, 

224 
Casehardened versus oil-hardened gears, 

101 
Casehardening, no, 180, 211 
alloy steels, no, 204 
American Society for Testing Mate- 
rials' methods, 201 
boxes for, 189, 228 
by carbonaceous gas, 211 
by gas, furnaces, 215, 223 



273 



274 



CASEHARDENING — FORGING 



Casehardening, by gas, steel to be used 
for, 224 
carbonizing in the furnace, 195 
depth of hardened surface, 207 
for colors, 209 
furnaces, 183 

furnaces, Giolitti, 213, 215 
in cyanide, 203 
local, 204 
of gears, 226 

packing for carbonizing, 194, 206 
packing materials, 191, 205 
Pennsylvania R. R. practice, 205 
quenching baths, 201 
reheating and hardening, 200 
reheating furnaces, 200 
results of hardening without reheat- 
ing, 202 
straightening work after, 208 
to clean work after, 208 
variations in methods, 198 
Cast iron, hardening, 176 
Castings, heat-treatment of steel, 175 
Centering tools, tempering, 76 
Chloride of barium bath, for heating 
steel, 51, 54, 136 
in electric furnaces, 128, 140 
Chrome-nickel steel, composition, heat- 
treatment, properties and use of, 98, 
106, 108, no 
Chrome-vanadium steel, composition, 
heat-treatment, properties and use 
of, 100, 108, no 
Cleaning work after casehardening, 208, 
Coal-burning casehardening furnaces, 

186 
Color method for tempering, 68, 70 
Colors, casehardening for, 209 
Composition of alloy steels, 104 
Composition of carbon steels, 104 
Cost of equipment for hardening gears, 

234 
Cost, operating, of electric furnaces, 

126, 134 
Counterbores and countersinks, temper- 
ing, 76 
Cracks in hardening, 21, 78 
Critical temperature, 4, 9 



Critical temperature, furnace for deter- 
mining, 10 
Current consumption in electric fur- 
naces, 126, 132, 148 
Cutters, for pipe-cutting machines, tem- 
pering, 76 
• milling and profile, tempering, 76 
milling, high-speed steel, heat-treat- 
ment, 91 
Cutting off high-speed steel, 90 
Cutting-off tools, tempering, 76 
Cyanide, casehardening in, 203 
Cyanide of potassium bath for heating 
steel, 50 

D eca l escence point, 5 

Defects in hardening, 78 
Derihon portable hardness testing ma- 
chine, 256 
Dies, pack-hardening hammer, 162 
scale on, after hardening, 170 
thread tempering, 6 
used in forging machines, hardening, 
171 
Drills, tempering twist, 76 

Electric arc heating for local hardening, 

153 
Electric furnaces, 48, 112, 128, 146 
advantages of, 118, 133 
barium-chloride, 128, 140 
current consumption, 126, 132, 148 
description of, 116, 130, 150 
for determining critical temperature, 

10 
heating gears in, 121 
metallic salt-bath, 128, 151 
miscellaneous types, 146 
operating cost, 126, 134 
Electric hardening, rapid method, 156 
Enamelite for local hardening, 179 
Equipment for hardening gears, 234 
Equipment for hardening-room, 182 

Fly-cutters, tempering, 76 

Forge-tools, hardening, 172 
Forging heat for high-speed steel, 90 
Forging machine dies, hardening, 171 



FORMED — HARDNESS 



275 



Formed milling cutters, tempering, 76 
Forming tools, tempering, 76 
Fuel oils, heating value, 30 
Fuels, solid, for furnaces, 49 
Furnaces, 26 

casehardening, 183, 200, 214 

casehardening with gas, 215, 223 

coal-burning, 186 

effect of heating in two, 19 

electric, 48, 112, 116, 128, 146, 150 

electric, barium-chloride, 128, 140 

electric, current consumption, 126, 
132, 148 

electric, operating cost, 126, 134 

electric, salt-bath type, 128, 151 

for determining critical temperature, 
10 

for heat-treating gears, 230 

gas, 26, 37, 45 

gas-heated barium-chloride, 51, 52, 54 

gas, temperature control instrument 
for, 45 

Giolitti, 213, 215 

kerosene, 32 

oil-burning, 28, 37, 45, 184 

over-fired, 29, 30 

pack-hardening, 163 

solid fuels for, 49 

tempering, 72, 76 

under-fired, 30, 31 

Gages, pack-hardening, 160 

Garrett-Tilley furnace, 41 
Gas casehardening, 211 

furnace for, 215, 223 

kind of gas used, 212 

steel used for, 224 
Gas furnaces, 26, 37 

advantages, 45 

for barium-chloride bath, 51, 52, 54 

temperature control instrument for, 
45 
Gears, casehardened versus oil- 
hardened, 101 

casehardening, 226 

casehardening box for, 228 

cost of equipment for hardening, 234 



Gears, for machine tools, heat-treat- 
ment of, 226 

furnace for heat-treating, 230 

hardened high-carbon, 228 

heating in electric furnaces, 121 

quenching baths for, 232 

tempering baths for, 233 
General Electric hardening furnace, 128 
Giolitti process of casehardening by 
gas, 213 

Hammer dies, pack-hardening, 162 

Hammers, pneumatic, tempering, 
76 
Hardened steel, scale on, 79 
Hardened surface, depth of, in case- 
hardening, 207 
Hardening, 3, 157 

alloy steel, 93, 204 

carbon steels, 1, no 

case-, 180 

cast iron, 176 

cast steel, 175 

cracks caused by, 21, 78 

defects in, 78 

effect on tensile strength, 21 

equipment for, 182, 234 

forge-tools, 172 

forging machine tools, 171 

gears, 101, 121, 226 

heating steel for, 19, 25, 26 

high-speed steel, 81, 84, 91 

in cyanide, 203 

local, 179, 204 

local, by electric arc heating, 153 

pack-, 157 

rapid electric method, 156 

spring steel, 174 

temperatures, limits for, 18 

use of magnet in, 6, 24 

vanadium tool steel, 171 
Hardness, and ultimate strength, rela- 
tion between, 247 

and wear, relation between, 245 

as effected by previous annealing, 19, 
20 

definition of, 236 



276 



HARDNESS — OIL 



Hardness, of metals, machines used for 
testing, 254, 256, 259, 261 

of metals, testing, 236 

of steel, 243 

red, 3 

scale, BrinelPs, 248 

scale, Shore's, 269 

scales, comparison of, 241 

tests, modern methods for, 238 

tests on worked or heat-treated 
metals, 242 
Heat, gaged by pyrometers, 54, 58 
Heat measuring instruments in general 

use, 57, 58 
Heating steel castings for hardening, 175 
Heating steel for hardening, 26 

by electric arc, 153 

in barium-chloride baths, 51, 54, 136 

in cyanide of potassium bath, 50 

in lead bath, 50 

in liquid baths, 49 

in metallic salt baths, 132, 136 

in two furnaces, 19 

length of time, 18 
Heating value of fuel oils, 31 
Heat-treatment, of alloy steel, 93, 104, 
no, 204 

of carbon steels, 1, 104, no, 180 

of gears for machine tools, 226 

of gears, furnace for, 230 

of high-speed steel, 81, 91 

of screw stock, 175 

of spring steel, 174 

of steel, Brayshaw's experiments, 15 

of steel castings, 175 

of vanadium-tool steel, 171 

use of electric furnace for, 112, 128, 
146 
High-speed steel, annealing, 85, 90 

cutting off, 90 

forging heat for, 90 

heat-treatment, 81, 91 

Taylor- White process for hardening, 

84 
tempering, 83, 84, 91 
versus carbon steel, 23 
Hollow mills, tempering, 76 



Hoskins electric furnace, 116 

for determining critical temperature, 
10 

Industrial Furnace Co.'s furnaces, 40 

Keep's hardness test, 239 

Kerosene for steel heating furnaces, 
32 
Knurls, tempering, 76 

Lathe tools, high-speed steel, heat- 
treatment, 91 

Lead bath for heating steel, 50 

Lead-tin baths for tempering, 74 

Length of time of heating for hardening, 
18 

Liquid baths for heating steel, 49 

Local casehardening, 204 

Local hardening, 179 

by electric arc heating, 153 

Magnets used in hardening steel, 6, 24 
Melting points of salts used in 
heat- treatment, 152 
Milling cutters, high-speed steel, heat- 
treatment, 91 
tempering, 76 
Mills, hollow, tempering, 76 

Nickel-chromium steel, composition, 
heat-treatment, properties and use 
of, 98, 106, 108, no 
nickel steel, casehardening, 204 
composition, heat-treatment, prop- 
erties and use of, 97, 104, no 
Nitrogen, effect of, on steel, 125 

Oil baths used for tempering, 70, 74 
Oil-burning furnaces, 28, 184 
advantages, 45 
modern types, 37 
Oil-hardened versus casehardened gears, 

101 
Oil tanks for tempering, 76 
Oil, tempering in, after pack-hardening, 
169 



OILS — STEEL 



277 



Oils, for quenching, 65 

for tempering, 74 

fuel, heating value of, 30 
Operating cost of electric furnaces, 126, 

134 
Over-fired furnace, 29, 30 
Overheated steel, restoring, 78 
Oxygen, effect of, on iron and steel, 123 

Pack-hardening, 157 
furnaces for, 163 

gages, 160 

hammer dies, 162 

quenching baths, 166 

tempering in oil, 169 
Packing for carbonizing, 194, 206 
Packing materials, for casehardening, 
191, 205 

for pack-hardening, 157 
Pipe-cutting cutters, tempering, 76 
Planer tools, high-speed steel, heat- 
treatment, 91 
Pneumatic hammers, tempering, 76 
Potassium-cyanide bath for heating 

steel, 50 
Pots for lead baths, 50 
Profile cutters, tempering, 76 
Pyrometers, 54, 58 

application of, 11 

calibration of, 57, 60, 233 

correcting thermo-couple, 60 

resistance type, 57 

thermo-electric, 56 

use of, 15 

Quenching, 8, 61 

Quenching baths, for casehardening, 
201 

for gears, 232 

for pack-hardening, 166 

for steel castings, 176 

list of different kinds, 65 

oils used for, 65 

receptacles and tanks used, 66 

temperature of, 64 
Quigley furnace, 47 

Reamers, tempering, 76 
Recalescence point, 5 



Red hardness, 3 
Resistance pyrometer, 57 
Rockwell furnace, 44 

Salt-bath electric furnaces, 128, 151 

Salt baths for tempering, 73 
Salts used in heat -treatment of steel, 

melting points, 152 
Sand bath for tempering, 71 
Saturated steel, 2 

Scale on dies when hardening, to pre- 
vent, 170 
Scale on hardened steel, 79 
Sclerometer, Turner's, 238 
Scleroscope, Shore's, 238, 261 
application of, 264 
description of, 263 
hardness scale, 269 
Screw stock, heat- treatment of, 175 
Shore's scleroscope, 238, 261 
application, 264 
description of instrument, 263 
hardness scale, 269 
Society of Automobile Engineers, heat- 
treatment methods for carbon and 
alloy steels, 104, no 
Spring steel, heat-treatment of, 174 
Steel, action of carbon in heated, 121 
action of nitrogen on, 125 
action of oxygen on, 123 
alloy, composition, heat-treatment 

and properties, 93, 104, no 
alloy, casehardening, no, 204 
carbon, annealing, 79 
carbon, Brayshaw's experiments on 

heat-treatment, 15 
carbon, composition, heat-treatment, 
properties and use of, 95, 104, no 
carbon, hardening, 1 
carbon, tempers for tools made from, 

76 
carbon versus high-speed, 23 
castings, heat-treatment of, 175 
chrome-vanadium, heat-treatment, 
properties and use of, 100, 108, no 
effect of difference in composition, 1 
effect of heat-treatment on, 2, 103 
hardness tests of, 243 



278 



STEEL — WATER 



Steel, high-speed, annealing, 85, 90 
high-speed, heat-treatment, 81, 91 
high-speed, Taylor- White process for 

hardening, 84 
high-speed, tempering, 83, 84, 91 
nickel, heat-treatment, 97, 104, no, 

204 
nickel-chromium, heat-treatment, 98, 

106, 108, no 
non-magnetic when at hardening 

heat, 6, 24 
overheated or burned, 78 
saturated, 2 
scale on hardened, 79 
spring, heat-treatment of, 174 
supersaturated, 2 
tungsten, Brayshaw's experiments on 

heat-treatment of, 15 
unsaturated, 2 

used for casehardened parts, 96, 181 
used for gas casehardening, 224 
vanadium tool, heat-treating, 171 
Straightening work after hardening, 208 
Strength of hardness of metals, relation 

between, 247 
Strength, effect of hardening on tensile, 

21 
Strength of heat-treated carbon and 

alloy steels, 105, 107, 109 
Supersaturated steel, 2 

fanks used in quenching, 66 

Taps, high-speed steel, heat-treat- 
ment, 91 
tempering carbon steel, 76 
Taylor-White process for hardening 

high-speed steel, 84 
Temperature, controlling instrument 
for gas furnaces, 45 
critical, 4, 9 

critical, furnace for determining, 10 
for hardening, 3 

for hardening high-speed steel, 81, 91 
for hardening, limits for, 18 
measuring instruments for, 54, 58 
of quenching baths, 64 
pyrometers for measuring, 54, 58 



Tempering, 14, 68 

baths, 70, 74 

bath for gears, 233 

by the color method, 68, 70 

carbon and low-tungsten steel, 21 

furnaces, 72, 76 

high-speed steel, 83, 84, 91 

in sand, 71 

lead-tin baths for, 74 

oil, after pack-hardening, 169 

salt bath for, 73 

steel castings, 176 
Tempers for carbon steel tools, 76 
Tensile strength, effect of hardening on, 

21 
Testing hardness of metals, 236 
Thermo-couple, correcting, 60 
Thermo-electric pyrometer, 56 
Thread chasers, tempering, 76 
Threading dies, tempering, 76 
Time of heating for hardening, length 

of, 18 
Tools, carbon steel, tempers for, 76 

hardening forge-, 172 

high-speed steel, heat-treatment, 91 

used in forging machines, hardening, 
171 
Tool steel, see "Carbon steel," and 

"Steel" 
Tungsten steel, Brayshaw's experiments 

on heat-treatment of, 15 
Turner's sclerometer, 238 
Twist drills, tempering, 76 

Under-fired furnace, 30, 31 
Unsaturated steel, 2 

Vanadium-chromium steel, composi- 
tion, heat-treatment, properties 
and use of, 100, 108, no 

Vanadium tool steel, heat-treating, 171 

^Vater annealing, 89 

Wear and hardness, relation be- 
tween, 245 



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